I only use enamels, thinned with lacquer thinner--both Testors Model Master and WEM's Colourcoats. I used to use Floquil's Dio-sol, which was pretty strong.
Spraying with an airbrush, I've never had trouble with melting plastic. In fact, "hot" thinners cause the paint to "bite" the plastic, with less chance of pulling up the paint, especially if you mask one color in order to paint another.
Bristle brush painting would probably be a lot different.