Hex, Whenever you have two different materials coming into contact, the only true answer is to experiment with a small patch first. Different manufacturers of similar products all could have material that varies in some way. But no, they shouldn't stick to each other.
As to not needing a mother mold if you make a mold of polyester resin, yes, you would still need a mother mold if you were casting expanding foams into it, like you seem to want to do. The expansion of the foam would force the two halves apart otherwise. A resin mold would be inflexible, and unless you were casting tapered candles, it would have to be a two part mold to be able to free the casting later.
As to what you could use to cast in it, I would think anything, as long as you use the proper release agent.
Here's a neat site to look through on some of the basics, and materials available. I haven't dealt with them, but lots of good info. Vlad >>>>>>>>>>>>