Well Dark Angels can swing both ways.

...same with Tau and Eldar.
IG and Sisters should be teamed together and probably Fists and Wolves but sometimes Wolves will take up odd fights so there's a little flexibility.
As for the warzone, you should definitely check out the
FW:Horus Heresy (Book 4) Conquest Campaign system... you can create warzones that totally follow your backstory. Check my blog for more info if you're interested.
But you could do something like: On the sun scorched planet of Eseark, all weapons with the
Gets Hot rule overheat on a roll of 5 and 6, additionally flamer weapons have the
Soul Blaze rule (and/or
Torrent if you really want to big).