So from what I've read, it seems icing a cake with cream cheese icing is a no-no when covering the cake with fondant. What about a cream cheese buttercream? I make mine by making a traditional swiss meringue buttercream and adding softened butter at the end. Will this cause the fondant to slip and slide? Is there something I can do to firm up the cream cheese icing a bit? Also if I do use a cc buttercream, I will fill with traditional cream cheese icing because I know that is what my client wants. She is a cream cheese icing aficionado.

We do it all the time. I make our cream cheese frosting by adding softened cream cheese to our standard IMBC. I just make sure the cake is really cold before putting the fondant on.