How dare flying Mario not be a favorite power-up, and how dare you get such a slick finish.
After painting a figure, do you have to go easy on the joints (i.e. no more play time)?
Haha, I guess I prefer the old powerups like the Fire Flower or Super Leaf.
It depends on what you do. Posing them in different positions is fine, as long as you aren't too rough you should be good. Sometimes you may have to file down the joints where either the arms or legs bend to avoid paint rub, but it seems with these figures it's not necessary. There is also paint sealant you can use which will prevent the paint from chipping off.
For example, the customs I did of Flying Mario, original Fire Mario and Luigi (white overalls) I disassembled the figures by popping off the head, hands and legs. You can do this by placing them in front of a space heater for a few minutes. The plastic will become soft, making it easier to remove the parts. I masked off the arms/shirt and shoes and spray painted. I removed the tape and did touchups if the paint overlapped. The arms are free to move around. The overalls should be fine as they were spray painted. When moving the knees there isn't any paint rub.
With the customs of the Bros. in their original colors or SMB3 Fire Mario, I used a base figure where their overalls were already red or green. I did this as it's easier and less painting. I swapped the heads with ones where the hat was red or green so it matched the overalls. I just repainted the arms, and again no noticable paint rub.
Sure, a figure may look good when finished. The most important thing is prepping it and sealing it properly so it stays that way.