Thanks for your replies!

There is no reason not to PC-ify a recurring NPC you like (unless there are! Check with your MC, he might need that NPC for something)

Well, I'm the MC, for starters, so that's not an issue. I've been trying not to need specific NPCs for anything, especially as they can be turned into Allies at the drop of the dice.

I guess my question really had more to do with the nature of Allies. They're a kind of resource, right? My instincts are not to let one player make another player's PC's Allies into his/her new PC, but if a player wants to do it to his/her own PC's Ally, then that's okay. No NPCs have been made Allies of multiple PCs yet in this game, though I'm excited for that to start happening.

As a side note, I notice that the description of 'retire to safety' necessarily involves creating a new character. Does that imply that, once you're running multiple characters, you're gonna be running multiple characters for the rest of the game? My instincts say no, but I am trying hard to run with the rules as written.

edit - I suppose your characters can get killed off, so that can shrink your numbers.

Two of the advanced moves (retiring a character, creating a 2nd character) involve creating a new character to play. I'm wondering how other groups have gone about this.

Is it generally considered cool to take an existing NPC and claim it as your new PC? How about if the PC is your Ally? Someone else's Ally?