Okay, here we go. I'll spoiler the images as there are around 28 throughout this post for various things, I wanted to be as thorough as I could (though I did forget to take a couple of minor shots, being me!)
Here's a draciel base with no SG, and two balls. As you can see, the base has poles. You cannot fit more than one metal ball in each pocket due to these poles.
You can also see the holes of the base. These are where metal balls were pushed into for the way people used to put 6 balls in it in WBBALand. Of course, this required a size of metal ball takara never used in the beyblade product line during plastics at least. I own all metal ball parts, though I've not tried Draciel G or V2's due to being un-removable and therefore illegal, they both look too small (G's look about the size of Gaia Dragoon's/Draciel V's, V2 slightly larger perhaps but still too small).
Here's it with SG Oil Ball. As you can see, SG oil ball blocks off the holes, and as I'll show you later, is the only thing preventing the balls from moving out of the pockets. It covers the poles too. This prevents any more than two metal balls.
Here's a picture of SG oil ball disassembled. As you can see, the platforms serve no structural purpose, and only seem to be there to prevent ball shifting or addition.
Now onto the various non-stock setups.
First off, here's one with Wolborg 2's SG and two metal balls, a side on just to show that everything is in order.
Here's what happens with wide defense if you shake it around a little. AR Removed so you can see it (also I didn't get a good shot with the AR on, but they're in the same position they were in with it on.) As you can see they can move around very freely. That said, exiting the pockets from here isn't too likely in battle as it takes a fair bit of movement to dislodge them, but I've had it happen against some of the more powerful attackers a few times.
Here's with a 10 Heavy, held upside down to show how far they can move with that. They can exit the pockets but unless your AR is loose there's no way for them to move around.
Okay, now onto the stuff that shouldn't be legal:
Here's the main method of fitting four metal balls into the base. One in each hole the SG's triangular protrusions (that you can't see clearly here, they are visible in later shots) don't obstructBad balance, and as you'll soon see, it's very questionable in terms of legality.
Here's what happens when shaken with wide defense (heavy WD's are less prone to this). As you can see on one side both balls have exited their positions. Didn't remove the AR this time, as doing that causes balls to fall out.
That may well be all fine and good, except it opens the gate for this kind of stuff. A few pictures here, showing grip base and BB Semi Flat with extra balls inserted through similar methods. In the case of grip base, the two balls shown are held in place perfectly. It takes a little effort to get them under the WD holes but they stay there.
BB Semi flat is included to show another example of how it's done (it's easier to do with four layer bases but a whole bunch of 5-layer ones can do it too), this is with a 10 balance WD. Sorry for the unclear image but there are two balls on each side. There's also a side-on of grip base showing slight WD Elevation and also a picture of SG Semi Flat on a launcher to show it can be attached (the balls must be kept out of the way of the protrusions launcher prongs attach to).
Here's another method of adding two more metal balls. It's one of the two methods required to use six metal balls, when used with the previous method. It's also the only four-ball method that prevents everything floating around.
This first image demonstrates where the ball sits. This is the only way of having two balls anywhere that could be considered "in the pocket", though it sits on the pole.
Here's a side-on showing this method used in the less-floaty six ball method. As you can see, the SG holds it in place but is slightly elevated, putting strain on the base clips. It's easier to push the second set of pocket balls in afterwards, otherwise you have to angle the sg to put each clip in and so on. The pole-sitting balls also push the balls in the pockets down- and outward, putting extreme strain on the pocket holders. You'll see some white marks on one of mine later, those weren't there this morning.
Here's two side-on shots of this six-ball method assembled, showing the gap it leaves.
The first picture in this spoiler shows a wolborg sg and shaft in fortress base with two balls. The second shows with four balls using the above pole-sitting method. As you can see, it sits lower. The angle doesn't demonstrate how much lower it sits all that well but it's very noticeable in person.
Here's the other method for getting 6 balls in there, the two additional balls sit above the pockets, next to the SG instead of below it. This is a position that the balls in the holes can sometimes move into, and if done without balls in the holes, they tend to move into the holes by the end of a battle (often during launcher attachment). This puts less strain on parts than the previous one, so it's arguably saner. Arguably.
Here's this assembled with 10 Heavy. As you can see it's quite a gap. Poor AR choice to show it though, but eh. Not sure what SG I was using, as I can't see the tip it was probably either an HMC and dragoon v/v2's tip or wolborg's sg/shaft.
As you can see, the launcher didn't go on well. One of the balls didn't move aside and jammed in place, leading to this.
From here on, I throw all restraint aside. I've combined the various methods into a method I call "cramming as many balls as you can wherever they'll fit", though really it is just a combination of the above methods.
This first setup uses Gaia Dragoon's smaller balls (roughly the same size as Draciel V's). I think there are about 8 of those small balls hidden in there and I later managed to fit 10 in total, which is where I ran out of them, while I have two more in the SG's of my gaia dragoons, it wasn't worth the effort (the two extras I already had were from a broken draciel v, but they're the same size).
This is, of course, in addition to 15 regular sized metal balls. Two Gaia Dragoon balls and one regular one are in the HMC, which is not considered legal but I was really seeing just how "ballsy" draciel f could get haha. Ariel's tip is heavier than Dragoon V2's, even though it means only using one small and one large ball in the SG, the total is still heavier, if anyone was wondering. It is launchable but usually the small balls will fly out, so it's a safety hazard.
Back to a more "reasonable" setup, here's my 14 regular ball setup. First from the top, as you can see four balls float in the base on each side, basically using the "above pocket" method. It's not like metal balls don't float through these positions in above setups anyway. In addition to those, there's one using the first 4-ball method on each side, one in each pocket, and one on each pole below the SG.
The pocket balls are best seen in this underside pic. As you can see I'm using Dragoon V2's tip (Ariel 2's is heavier, I just couldn't be bothered removing Dragoon V2's.)
And here it is on a launcher, side on. As you can see, it attaches just fine. This is why one of the balls on the gap side of the SG is piled up, to make more room for the prongs.
And here it is spinning. Early detachment is definitely a hazard with this setup, but only happens about a quarter of the time as long as you're careful with the attachment. Between that and the weight it didn't move very fast haha.
So, onto rulings:
The most sane four ball setup, which is resting them on the base holes, would still open the gate for up to 12 balls packed into the base, and both that and the above-pocket method open the gate for cramming metal balls into every other base that can fit them. The only one that doesn't open that up, and the only setup that allows the use of SG's other than Oil Ball without ball shifting on any WD, is the most questionable of all of them due to the extreme strain it places on structurally weak parts (pocket support and base clips). Any method of using six throws all pretense of sanity aside, and opens the gate to just jamming balls into every orifice they can possibly be forced into.
Of course, as I said, removing the ability to change SG's would remove the small use the base has. I think we'd be okay to allow that but limit it to two balls in the base. It's a blader's choice if they want to use a weight disk that risks imbalance due to ball shifting, and it's generally not that huge a risk, fortress base has appalling stamina whatever you do with it anyway.
Enough detail for you? I put my fortress base on the line for this, so yeah :3