When we were planning a Wishes wedding at EPCOT, we were told that there might be ceratin foods that would not keep well in the warmers, and with the kitchen so far away, the chef had the right to tell us no if there was something they didn't think would keep well. The things I remember we definitely couldnt do were ice cream, frozen cocktails and I think mousse filling in our cake. I believe there was one entree we were interested in that also got the "no go" but I can't remember now what it might have been. In general they're happy to let you have what you want if they can do it and have the quality be ok.