This is interesting. I'm a brand-new ukulele player, coming from many years of (classical) double bass and a few years of clawhammer banjo. I started with Aaron Keim's fingerstyle book, which teaches PIM, but I've also been working on some books and exercises by Rob MacKillop and Samantha Muir, who use PIMA. Maybe this is foolish, but I'm trying to learn both. Aaron's arrangements work so nicely with his system that it's best to just stick with it. There is one exercise in Aaron's Ukulele Handbook that works great in PIMA, too, though. However, for the other exercises, I'm trying to use all 4 fingers.
Thus far, I haven't found it too confusing largely because I can sort of view the AK system as just a subset of the other one - I don't feel like I have to break PIMA rules or guidelines to play his arrangements with his system. However, I think I'd have a harder time playing the MacKillop or Muir exercises unless I used all 4 fingers. I'm hoping this strategy doesn't cause problems in the future, but so far I feel I am progressing on all fronts.