I have to make it the day before so do I refrigerate it or leave it out? Is there something I can add to the whipped cream to stabilize it? What does everyone else do in this situation?
There are probably more people on here who can tell you the science behind it but I personally won't do a mousse filling in a fondant cake. I always leave my fondant cakes out so I won't take the risk with mousse. Also I find the mousse to be not stable enough for the weight of the fondant. I'm sorry I couldn't help you more!
You put it in the fridge so you don't give the people eating the cake food poisoning from a perishable filling being left out.
With a super-stiff dam of buttercream icing, you can use mousse as filling. I do it with cookies 'n cream mousse in chocolate cakes.
You can add gelatine to the whipped cream, it stabilizes it. I second Cakepro - you need buttercream dam and refrigerate - not only to hold the weight, but because some fondant sweets from the mouse. Some fondant doesn't like mouse filling - to be on the careful side - don't let touch the fondant the mouse directly. Peek in the fridge after 1/2 hour or so - sometimes you need to crank it on full to stop the fondant sweeting.
Fridge the fondant. I do it all the time.