Surface effects could be easily balanced by just having any fire surface have 1 turn duration and do less dmg. Same with reducing explosion dmg so it doesn't outright kill anything. Now for stuff like grease it should be permanent unless the player or enemy decides to ignite it for some quick area damage and could also serve as some way to clear it fast if the surface is too big. Just program the A.I. to shoot it with fire spells, alchemist fire or fire arrows if a player decides to block a door/pathway with grease unless it's some other kind of monster with no way to produce fire.
I think 1 turn is too short, it should be 2-3, but not more. Enemies should try to avoid fire
