Can you have more than one skin?

[Burnsaber]

I once tried modding a spell that gave barkskin, stoneskin, ironskin and invulnerability in a single spell. IIRC, only the two "top" spells in that spell took effect and the protection value was decided by the better spell. So the spell basically gave affected units the protection from invulnerability and the the negative poison resistance from invulnerability and the negative shock res from ironskin.

Based on that, I'd say that casting SoG wouldn't overwrite your previous stoneskin/ironskin although it would give the fire resistance malus. If I were you, I'd test it first though before trying it in MP or something.