Pokemon Infinite Fusion - where to start?

look at the sprites before you commit to a fusion. some pairs have hand-drawn custom sprites and some are auto-generated and look pretty rough. sounds shallow but you'll be staring at it for 20 hours. the wiki's fusion dex shows which is which.

The order of the fusion matters more than which starters you pick. The head gives the primary type and leans the stats toward HP and the special side, the body gives the secondary type and leans toward attack, defense and speed. So try both orders before you commit, the same pair can be great one way and useless the other. Don't fuse too early either, you can unfuse but it's nice to have both evolved first. And back up your save folder before you update the game, it's a fan project and updates do occasionally break things.