To be honest, I don't have a codex or anything so don't know options.
But I do have a bunch of Guard figures and a slew of DKoK and a slew of renegades and a bunch of Chaos, so I was originally going to make a "traitor Guard" force to compliment the Chaos force... so I suppose I could make the Cult figures' uniforms and such match so I have flexibility.
I don't even play 40k and am dusting off models to maybe start painting again in between all my other jobs/tasks. But I reckon were I to play down the road, having the flexibility to be able to field a GSC force with a slew of "counts as" models could help. Or vice versa, I don't know.
I see this GSC as forming up after Chaos took a world or system or something. They nabbed it, thus the "traitor guard" link and then went out-system to conquer more stuff. And, as we all know that when you leave doors open the bugs can get in... ha ha.
Any advice/thoughts/ideas?