I've recently started using some similar pens / felt markers ( http://www.mscdirect.com/product/details/57310559
). Although markers and pens tend to have significant differences in the way they show on media, when it comes to plastic or writing over paint it all depends on what the ink is designed for. It's safe to assume that unless it specifically says "for use with plastics and/or paint" that it's probably not going to give good results.
Even after it dries, whether it's marker or pen, most inks will still smudge easily on plastic. Paint I'm not sure of: it would probably depend on what paint you're using. Something like watercolor would probably be reconcilable to draw over with pens, but if it's acrylic or enamel (which I assume dries harder and more glossy) my guess is that it won't do so well, assuming it writes at all. A marker-pen like in my link would probably work best (ink flow is better with markers and you can still get a great precise line with the 005 size) but it may still smudge.
Hope that helps!