I use many different ranges for my 15mm fantasy chaos....
Demonworld, obviousl. Look at their goblin range, they do a great mutant pack.
Eureka (Fighting15s) do so me great 10mm beastmen that fit really well as 15mm figures, I use them as a demon horde.
Blood Moon make a small range of demon type figures but I also use their goblins as a demon minions.
Pendraken 10mm can fit in well
GW, yes I do use them...chaos familiars (expensive now but very good), Nurglings very nice, especially the old metal ones.
Original Ral Patha Battelsystems 15mm goblins, again a different warband
There are few examples on my blog
http://warrenswargamerantings.blogspot.com