You can replace epoxy with polyester resin for any of the places you are COATING something, not for BONDING parts.

You can replace epoxy for bonding with other glues in some instances. Where and when is a matter of experience. until you have experience with the various adhesives, it is best not to experiment on a model.

I tend to replace epoxy as much as possible when building. I also avoid using CA as much as I can and use yellow carpenter's glue for most of my construction. (it works fine, its light and strong, and I can build a 4*40 in 3 days easilly with no CA except for putting in the CA hinges.) I find I save an average of $4 in glue using the yellow vs using the CA on a .40 size aircraft. (and I always come out below the kit's listed weight...)