Hey Derek,
I already posted on the link provided by Mr. InfestedKerrigan, but I'll sum up quickly my feelings on mixed daemon lists. With the toughness of Nurgle, the melee prowess of Khorne, and the psychic supremacy of Tzeentch you are looking at a very balanced and effective army that can deal with a lot of different situations. I would recommend a solid core of a few Nurgle units to establish a sticky, objective holding portion of your army and supplement it with the punching power of Khorne and Tzeentch. As a primarily Nurgle player I love nothing more than my opponent trying to kill off even a single unit of Plaguebearers and me passing all of my Inv. and FNP saves. If you choose to use the Daemonic Incursion list be prepared to have to pick up a lot of different units which the Core and auxiliary choices force you to use. Just decide what Core/ Aux choice looks the most enticing to you and go for it. Happy collecting/gaming and let us know what you need help with as you start your descent into darkness and eternal damnation.