1) wall plaster melt foam? 2)what glues dont melt foam?

also what glues wont melt the foam? because i need to base the foam onto some thin plywood to give it some strength so i wont snap when moved or lifted.

Elmer's type white glue and related products, such as wood glue.

1) Shouldn't. You can get some pretty cool textural effects on the foam with laquer thinner, but use sparingly. It can be difficult to control. A wire brush is good for forming and making terrain mods. Keep a vacum handy though as little bits of foam get everywhere. A heat gun can also smooth out forms that may be too angular. Again use sparingly. Before applying your plaster, give it some texture for the plaster to hold onto. Wall compound works pretty good and plain old spackle works just as well.

2) As Al stated white glue works pretty good. The same company that makes liquid nails also makes a product specificaly for foam (for gluing foam insulation panels). This stuff works really well.

Yeah, white glue is the best think. Make sure you roughen up the base and/or even drive a few tacks in to give the glue some purchase. Over time white glue will dry out and the whole thing can detach from the base. I found a product called "Bulldog multi grip" works very well.

i use styrofoam on just about every dio i do and i found out that after you carve your basic shape paint a thick layer of epoxy paint to seal it .then you can use any paint you want without melting it ,but as for glue i use HOB-E-TAC made by woodland scenics this stuff never dries and stays very sticky so if a year down the road you want to move something just pull it out and restick it somewhere else