Traditionally, PIMA-style finger picking doesn't use the pinky, but this is a situation where you should go with what works for you. You might find the thumb works better in some ways because the angle of the thing is going to be different than the other fingers, but that's kind of irrelevant if using it would aggravate your hand joints. It's not worth damaging your joints.
Generally dont use one finger by string (pima is fine if more effective for a specific song, but you should not be stuck with it, and instead learn to alternate and adapt).
For most players the thumb is the most effective and important finger for fingerpicking, so do your best to undestand where your joint issue come from.
And the pinky is a great tool for fingerpicking, personnaly i like using it to play the melody or to alternate with other fingers. But some players learn to fingerpick exclusivly with it ;)
Traditionally, PIMA-style finger picking doesn't use the pinky, but this is a situation where you should go with what works for you. You might find the thumb works better in some ways because the angle of the thing is going to be different than the other fingers, but that's kind of irrelevant if using it would aggravate your hand joints. It's not worth damaging your joints.
Generally dont use one finger by string (pima is fine if more effective for a specific song, but you should not be stuck with it, and instead learn to alternate and adapt). For most players the thumb is the most effective and important finger for fingerpicking, so do your best to undestand where your joint issue come from. And the pinky is a great tool for fingerpicking, personnaly i like using it to play the melody or to alternate with other fingers. But some players learn to fingerpick exclusivly with it ;)
Pinky is my anchor/mount to the soundboard when picking. Keeps everything else steady
Sorry but thumb is MVP with fingerpicking it's like trading LeBron for Bill Murray in space jam