Unless you're looking to add another caliber, I'd either get a 30SF to go with your 21SF or a 27 to go with your 23. I carried the 23 & 27 combo for years. Being able to use the larger magazines in the smaller gun, if needed, is nice. The 27 was actually easier to shoot and more accurate than the 23. This is something that people seem to report consistently.
With the 30SF, because of the large base on the magazine, you may not need to add an extension. With the 27, you can get a Pearce PG2733 that gives you a +1 on the magazine and is angled so that it doesn't add anything to the backstrap, which is where it would be an issue for concealment.
As for the .45 GAP, I currently carry the 37 & 39. It's a great shooter, very accurate, everything you could want in a .45, just a little smaller and easier to grip. You can get a Pearce PG39 for that one. You can put the 2733 on it, or even the PG26, but they won't add anything but a little room for the pinky.
With the exception of the 29 & 30, which are the 10mm & .45ACP, all the rest are the same size, regardless of caliber. Well, the .45 GAP slide is a little thicker, but the frame is the same as the 9/357/40.
If you are looking to add a new caliber, the 33, which is the subcompact .357Sig would be a LOT of fun!
