My players have decided the best way to fight a nurgle greater daemon is with a tzeentch greater daemon. so their plan is to make pacts for knowledge to gain the know how to be able to summon one of the big baddies. My question is if anyone has made rules for greater daemon, or even just herald level, summoning.

By RAW there is no difference in the summoning process, but the power level of daemon affects Daemonic Mastery test.