I gave up on trying to get the micronaut into the FW suit, and just stuffed it with tissues. I took paper clips, and bent the tips into circles, which I then inserted into the holes of the Godzilla suit feet. I made an oval-shaped loop with the rest of the paper clip to insert into the hole at the bottom of Godzilla's legs. Feet stay on just fine. If I had him with me (I'm in Tacoma, WA and the figure is in Beaverton, OR) I'd take pictures and show you.
I also have the "Little Braves" Toto Gamera, which I successfully got the micronaut into after many many tries, but there was no way to actually pose it without something popping out or breaking. So now the figure is just stuffed with tissues, and he stands up just fine.
I'm not sure what's up with the rubber material though. Since the arm holes on the FW Godzilla are too big to have the hands fit snugly into, I took the hands from the GMK Hyper Godzilla figure and fit them into the FW Microman Godzilla figure's arms, with just the hands sticking out. Pretty much the same color, looks just fine. (the microman FW Godzilla's hands DID seem pretty large). But here's the thing: the hands melted. The rubber of the Hyper Godzilla figure and the rubber of the microman Godzilla figure reacted in such a way, causing the Hyper figure's hands to melt. They became stretchable/pliable. The rubber on the microman suit, on the other hand, became fairly hard. Not sure what happened, but I don't want to try it again. And I have a poor handless Hyper GMK Godzilla.
Completely forgot the question.
Oh, right. I'd say give up entirely on getting the little dude in there. Plus, the micromen are capable of so many dynamic poses that they're just incapable of doing if they're crammed inside the suit. Don't limit their potential

These suits are pretty awesome on their own (without being able to be posed), and stand up just fine and maintain rigidity when stuffed with tissues.