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Can you use the normal cadian boxed set or not>
Well, if would take a little converting, but you could use them just for cultists if you file off the aquilla (imperial eagle thingy). still, they would look a lot like guardsmen, but aright if painted suitably.
You could then use the plastic genestealer heads and the cadia bodies to make the hybrids, perhaps even using the gaunt box for other stlye heads and arms, the bioweapons could look good on normal guardsmen rather than lasguns. Sanctioned psykers and genestealer parts could be used to make a magus.
bottom line is that, yes you could use the Cadia box set, but you would need other boxes for the bitz, and at then end of it you will have lots of leftovers that you won't use.