I'm doing this because I heard these stories about epoxy compatibility but never really knew the in-depth story. I'm finding that many people who sell glass fabric don't really know either.

Most resellers of fiberglass cloth don't tell you what the finish. The finish is what chemically bonds the resin to the glass fibers. It can make a huge difference in how the fabric handles (eg softness). Simple answer is virtually all the common fabric finishes are suitable for epoxy. For instance most Silanes and Volans are compatible with epoxy. Polyester can only used with some (ie some of the Volans). Volan by the way gives the green tint to old fiberglass cloth. Un-finished fibeglass off the loom (ie greige) has an oily coating is not compatible with either.

Thayercraft has a great article on fiberglass cloth finishes and weaves.
http://thayercraft.com/finishes.htm

Happy fiberglassing!