Following a quick trip to GW today I now have most of the parts to put together what I think will turn out to be pretty good first generation hybrids. I'm going to use Crypt Ghouls in the main, mixed in with extra arms. Here's an early blu tacked experiment:
On a trawl for internet inspiration I came across this great old diagram showing the different generational stages:
I'm not going to follow this exactly but it does give a good indication of the levels of mutation at each stage. Thanks to the gods of eBay I have another genestealer sprue on the way with plenty of heads on, some of which may well make it onto first generation models.