You'll want the 5mm bore mod1 pinions. The hardened ones can be used with a steel spur.
http://www.rc-monster.com/proddetail...1pinion&cat=40
I think 14-16 is around what you'll want for racing, depending on temps.
http://www.rc-monster.com/forum/showthread.php?t=6832 Don't know if you've seen this thread, but he's using a very similar setup to what you'll be using. He does have extra capacitors on the mm which he says help keep the temps down.
I don't know how common it is for the motors to come apart, but it does happen. I've seen people use long screws and a plate to clamp the motor together, but it won't have the heatsink. You were going to do long races with it weren't you? The feigao will probably need heatsinks for long races.
The xl and XL will be the same thing.
Some people had luck using the older mm's on 5s, but it seems the newer ones are a bit different and don't work above 4s or 12 cells nimh.