We have no reason to belive that the Lords of Decay would have any interal conflict. However for the Lords of Change we do as they are often discribed as having the emotional stability of insane children. Puting more than one Lord of Change in the batlefield would cause problems let alone 100 of them. However, as I acowledged above, ofcourse the Lords of Change would win, but, as I said, if Tzeentch sent 100 Lods of Change against the LofDeay we canot assume that Nurgle would do nothinng. Mortarion and a handfull of Great Unclean Ones would whipe the floor with them for sure. Mortarion being the primarch that exemplified the vitality, and determination of the Emperor himself and now is a deamon prince of Nurgle, who is all about that. Mortarion would have no problem taking out the bulk of the enemy force, just like Sanguinious took out all those Bloodthirsters by himself, and the rest would probablly flee. But again as I said this is not entirally a fair scenario as it has been cleared up it has to be a balanced, troop based, force.
I supose the ultramarines would win because of plot armor but I'm with Silence. Not in a logical scenario. And no I don't see Tzeench doing this either.