Not always. I was able to put Re issue Ginrai into Overlord, PM Prime, and Darkwing. It did not fit into Dreadwind at all, and kinda fit into Getaway, but wouldn't stay in.
I think they're supposed to in theory, but don't always. I can't get Doubledealer's purple bat to fit with Powermaster Prime, which is probably for the best as it would look pretty silly.
nope, the pegs on the legs are in different places on some of them and make it so that only certain ones activate the gimmick on each bot