I used enamels and acrylics on Airfix figures, you can even use oils, there's really not much you can do about flaking or cracking that is caused by bending the soft plastic--if the piece bends far enough, the paint will flake.
However, if you use the usual prep techniques that you'd use for styrene, resin or metal, you can reduce the likelihood that the paint will flake or wear.
I wash them in warm water, used to use a couple of drops of dishwashing liquid, but now I use the de-greaser Super Clean, dry them on paper toweling, and then prime with my usual automotive primer (Walmart's store brand du jour).
Hope that helps!
Brad