I think people have used chalk pastels for customising, but I really wouldn't recommend anything but those and acrylic. Oils are a lot more toxic and expensive, so I can't imagine them being all that well suited to painting plastic toys. :p And watercolor is made to be transparent, so again I think they'd get expensive or not cover as well.
If you already have them it can't hurt to try on some baits, but I wouldn't go out to buy new media unless you have other uses for 'em too! xD