a) Can't remember when during the post game but yes, the upgraded version of the Metal Slime disc will be available. b) After you beat the main game you will be given a new form of fusion that allows you to choose which traits your child gets from its parents, including body size. c) Hard to say since this game involves constantly getting new monsters. Being stat capped (no longer getting any stats on level up) is as far as you need to go. Even if you can't, you can always use your network coins or temporarily disable the game patching to get more.