RTF is a global modification that replaces all towns in the game.

It's not necessary for all units to be related to the Void. You just need to choose roughly the same color scheme, for example:
Alt-grade:
1) Battle Cabals (you can get away with just painting the armor)
2) Storm Elementals (because they are darker)
3) Mist Elementals (because they are darker)
4) Flame Elementals (energy elementals, blue, will go to the Simurghs and Djinns in the 2nd column)
5) Onyx Elementals (because they are darker)
6) Void Djinns (because they are darker)
7) Void Eagles (or something like that, but you'll have to give up on the Simurghs)
Upgrade:
1) Master Cabals (you can get away with just painting the armor)
2) Some kind of Hurricane Elementals (make them blue/light blue)
3) Ice Elementals (because they are lighter)
4) Energy Elementals (because they are blue)
5) Some kind of Lapis Lazuli Elementals
6) Medium Djinns
7) Simurghs
Something like that.