Like Jeff, I have mixed PollyScale and Model Master Acryl (not regular Model Master which is, IIRC, enamel based) with no problems. Hence the assumption - dangerous, I know - that the formulation is the same (or, at least, so similar as to not cause problems). FWIW, I have also mixed PolyScale and TrueLine Trains paints - both made by Testors - with no problems. Not sure what the current status of the TLT paints is. Some colours that I wanted were relatively easily produced by modifying a TLT colour with some appropriately coloured PollyScale.
Cheers,