I'm in a similar position, Love the Alpha Legion fluff and heresy era paint job & style but just not really into those GW cultist minis and a lot of the daemon warped units.
I'm going to use these as myu cultists
GCPS Troopers | Warpath
paint them in grey and red to contrast agaisnt my alphas, i'll fluff them into a subverted planentary defense force. If i want to make the army bigger and grow this toa full detachment i'll use forgeworlds renegades and heretics index to start bringin in vehicles and heavy weapons teams for them.
Another plan is to model two mk iv marines as a weapon team on a 40mm bases (one lascannon, one crouching with a plasma gun perhaps) to represent obliterators.
Lastly i'm eyeing up the Primaris reivers to convert (lose the pistols, add some chain axes) to represent Khorne berzerkers.
Lots of plans not a lot built sadly but am happy to send photos across as i get it done. Are you putting pics of your project up on a blog anywhere?