Skitarii Ranger and Sicarian Infiltrator stuff is in scale with each other. Swapping them around looks perfectly fine

The Sicarians are on a larger base size because of their kickass spindly and wide-spaced legs.
I'd agree that the rocket launcher might look a little out of place. I wonder if there are any bits that can be salvaged from the Skitarii kit to either make the base rocket launcher look more Ad Mech, or come up with your own kitbashed rocket launcher.
Oh, and Skitarii Vanguard heads to fit rather nicely with
GS Cults stuff, although to get the head to fit you have to shave off some of the back of the helmet and sometimes the little breather thing on the
GS Cults torso.
They're my mining guild/closet
GS Cult from my spaceport modelling idea
GS Cults heads on Skitarii should work with no modification at all