My guess is that its the styrene that makes it so aggressive on foam, it provides the nasty smell too.
If you coat the mold with a coat of shellac or lacquer then you can apply the polyester and it won't attack it because of the protection that layer of paint provides.
Some of the urethane foams are also resistant to polyester. Do wear the same charcoal chemical mask though, those urethane expanding foams have harmful emissions too.
Freeman Mfg & Supply Co sells lacquer as "Wood and Plaster Sealer" at freemansupply.com and I think it works well, their mold release wax on top of it works well too so its easy to demold the parts.
Uscomposites.com has great prices on just about everthing, and their 440 polyester and 635 epoxy resins work quite well and are quite cheap. If you're using polyester resin you picked up at an auto store or other retail store, the 440 is a dream in comparison. It has near water consistency so you can wet out quickly with low resin buildup, like 20+ minute working time before gelling, and can be sanded in 2-4 hours depending on mix and conditions. The store bought stuff is like 7 mintue work time when mixed at 3% and still needs overnight before sanding, and seems much weaker - that is not in side-by-side comparisions but I really don't like the stuff so its rare I'll even need to run out and buy some anymore.
Oh, and on your usage question, the epoxies are generally tougher (you can get very tough polyesters and vinyl ester too) but cost a lot more than polyester. The epoxies also don't have the styrene in them so they don't break down the binders used to hold fiberglass mat together, but I've used it and it still works ok.