The cultists aren't half bad but I dunno how it will work with points. Culstists are so cheap you could could fill your group to the max with cultists and still have 600 points over lol.
And that's would pretty hard to achieve as you would need to kill the champion since you must start with him. and then convert at least half the cultists to troopers for the list to be legal.
Even with a normal list you still need one non cultist per cultist. At least until the first cultists level up. You can't give them camo cloaks or anything other upgrades really so you're still wasting a lot of potential points.
Their not bad though I think, their melee weapons are pretty nice.
I'd forgo auto guns for auto pistols but some shotguns might be worth taking. Still I 'd prefer melee potions and have them run up with the champ
You would be lacking high impact weapons though so the best would probably be to have a gunner or two even and get them some nice big guns.
You could have two gunners, three cultists and a champ with the gunner both having auto cannons and still have almost a hundred points left for gear.
The two autocannons will be pretty nasty early on. I think I'd spend the extra points on more gear for the marines first.
The cultists need to survive the first three missions to become troopers and allow you to buy more cultists, so you won't be able to buy more guys for the first few games anyway unless some die.
100 points is enough to give each one some gear.