I've never had a problem using my pinkie. I would say I use it as much as my middle finger. And I use my ring finger second most, which sucks at the moment because I cut the nail too short and it's all in-grown. Very painful
I think I built my 'pinkie resistance' or whatever from playing guitar as a kid for a few years. I couldn't imagine not being able to use my pinky. For instance, in a major scale, how do you go from 3 to 5? That's a very comfortable index-pinky jump. Maybe just practise doing that or something?
As for the building up calluses thing, I play flats, so my fingertips aren't all hard, kinda soft yet tough... I can play pretty much all day, and the only fingers that get sore are my plucking ones. Maybe try using less fretting and plucking pressure and more volume?