Here is how I would build them (note these are just general skill ideas, not necessarily the order one should take skills or the only skills worth picking):
*Note* Just useing the normal Chaos builds works decently well with Nurgle guys (though obviously Rotters don't match up with anything)
Rotters
Normal Rolls: Kick*, Wrestle, Fend
Double Rolls: Guard
Stat Ups: +Mv/+Str/+Agi are all good.
*Only one.
You're only going to have a couple of these guys on the field at a time once you have all your positionals so they are probably best used to absorb hits on the LOS. That + Decay means they shouldn't live all that long which isn't a huge deal anyway as they don't need many skills for them to do their jobs. If you don't use them on the LOS then one of them should be your kicker just to free up a skill on your pestigors. If you should roll a stat up on your Rotters it's up to you if you take it. It makes your Rotter a good guy but Decay means he won't live long so it's a question if it's worth while.
Pestigors
Normal Rolls: Kick*, Block/Wrestle, Guard, Mighty Blow, Tackle, Stand Firm, Claws, Frenzy, Strip Ball
Double Rolls: Dodge, Jump Up, Diving Tackle
Stat Ups: +Mv/+Str/+Agi are all good.
*Only one.
I'm not a fan of making these guys into throwers/catchers. You need too many doubles/stat up to make it worth it really IMO (and you only have 4-6ish players on the field with agi 3 anyway... Not great options for throwing targets). Instead they make great mobile bashers since your warriors are going to be in the thick of it anyway and they aren't that quick on their feet. How exactly you build them though is tough to call, there are plenty of nice ways to do it.
Nurgle Warriors
Normal Rolls: Block, Guard, Mighty blow, Claw, Stand Firm, Break Tackle
Double Rolls: Dodge, Jump Up
Stat Ups: +Str Only
Standard bashing kit. I don't see much need to change it.
Beast of Nurgle
Normal Rolls: Stand Firm, Break Tackle, Guard, Grab
Double Rolls: Block
Stat Ups: +Str only
Standard "Big Guy" set. Stand Firm is a bit more key here however to keep them from pushing your guy away.