Like Pieter said, the main source of the track must be the boss/antagonist theme, and other sources used should also focus on enemies, dungeons, battles, bad situations, etc.
For example, a FF7 mix of "Those Chosen by the Planet" + "Those Who Fight", or "Those Chosen by the Planet" + "The Northern Cave", is fine, as long as the first theme is used as the main source. However, a mix using "Those Chosen by the Planet" + "Fiddle de Chocobo" will be discouraged (and would be weird lol).
As I said earlier too, tell Pieter or myself by PM or by posting here, and we'll figure it out.