For my Cultists, I mostly went with the Frostgrave Cultist kit. The hood heads are perfect for recreating the original Mordheim Cultists, the undead parts which come with it make for good mutation/possessed features, & it's an easy kit to work with. The choice of heads includes half-hoods with the lower face visible, full hoods, & hoods with attached masks (maybe useful for distinguishing Darksouls), & the zombie heads. Plus, its compatible with the Frostgrave humans & barbarians, opening more bits options. Oh, & there're metal Frostgrave cultist models, some of which are really nice.
For my Possessed, I used a Warhammer Fanatic (the one with his head in the stocks), & he's a bit taller, but not so much that it's a problem. Certainly GW miniatures are... chunkier, so you couldn't use GW arms on a Frostgrave torso, or vice versa, but I find they look good side by side. A unified paint job will do wonders for group cohesion.
Oh, & mutation-wise tentacles are fun & easy to model, & will ruin your opponent's day in-game. They're a very available bit, so I just fixed one where an arm should be, puttied up the gap, & painted it like bruised flesh round the join. Unfortunately I used quite a large metal tentacle (old Chaos Spawn one, I think), & the poor mutant had to be rebased to stop him falling over.