Hello and welcome to
40k!
I would consider getting rid of the Combi weapons for the
PM squads, and maybe switchero around a bit and get a small normal
CSM unit (you can still paint them nurgly) and man the Quad with. Another idea would be to grab a 10 man unit of cultists for it.
Generally most people (including me) don't pick the Gift of Mutation since it usually doesn't bring so much to the game. Sure its fun and all sometimes but mostly its pretty meh
You may also consider grabbing something with a bit more punch for your
PM Champs, like a fist or a Power weapon (they usually hit last tho -1 initiative vs
MEQ(Marine or Equal) and fists are kind of expensive). I find that they are sturdy enough that it might pay of to actually give them fists tho!
Another thing that I'm getting for my nurgles are Spawns, seems like a hell of a fun unit to me!