AFAIK only certain mutations are particularly cared for by even the worshippers of Tzeentch. Less tendrils and more wings, bits that breathe fire, feathers, talons. You know, the sort of thing that someone can be sure the lord of change has gifted someone with rather than the bog-standard shit your average mutant has. Either way, given that the ones marked for greatness by Tzeentch tend to be smarter than the average folk, we've got ourselves some good squad leaders.
Now, are you able to elaborate how further changes to them will risk the cult at all Tack?
Militia
By giving cultists some basic training, we can get them to do amazing things, like looking down a gun's sights, taking cover when someone shoots at them, attempt suppressing fire and covering their allies, spread out their fire against things that don't require concentrated fire to bring down, recognise the difference between soft and hard cover and recognise that pretty much everything is the former so don't stay in one place, and generally behave vaguely like soldiers with hints of competence and a survival agenda rather than a clueless gang of hooting thugs looking to prove themselves.
Flak Armour
While strides have been made in making our forces mobile, making them survivable hasn't worked out too well so far. Thus, plans are laid in motion to create armour roughly equivalent of the PDF/Imperial guard (Or just outright stealing it from the factory lines). While it gets a bad reputation as being worthless, this is mostly because it goes up against things that are far more powerful than autoguns or knives. Since knives and Autoguns are all both sides have down here for the time being, such armour would serve well with distinction.
Stubber Buggy
With the ability for fixing up Goliaths for service well in our grasp, development of light, armoured combat buggies is seen as a step forward. As a two-man, four wheel vehicle armed with a pair of stubbers (Effectively twin-linked), fairly well armoured for a vehicle of it's size and a hardened, bladed frontal plate for driving through enemies.
Space Marines: Alpha Legion
Having heard stories about the famous and dreaded space marines and their legionary counterparts, we have learnt that there are several legions that we could call upon for aid. Of course, the sorcerer has let slip about the fact that seemingly every thousand son we could call upon would be... well, mindless (And the sorcerer would rather not have any potential usurpers in the form of a legion sorcerer). To that end, this leaves us with limited options in where we source our marines from; Three other legions are worshippers of the other gods and the word bearers preach chaos undivided which makes them unsuitable for accompanying our cult. Of the other four, the black legion would usurp leadership from our sorcerer, the iron warriors wouldn't bother with the cult and the night lords are only useful as short-term mercenaries (And hate the chaos gods). This leaves us with only one choice for space marines: The alpha legion.
From what the Sorcerer tells us, these marines are rather.. devoid of personal ambition or goals, and are experts in the warfare which we must pursue to throw off the hated imperium and the xeno cultists (And more than inclined to educate cultists in these means as well.) While bannerless renegades are serviceable, a more controlled and discipline force of chaos space marines would help give the cult a firm backbone to expand from.