I want to use it as a cake filling for a fondant covered cake (haven't decided on the icing yet, suggestions?)
Will it be alright to leave the cake out for the next day? If not, I need suggestions for a new filling for marble wedding cake that can be left out.
i refrigerate fondant covered cakes...is that what you're worried about?
Absolutely 100% yes. Ganache (whether whipped or not) can be left out roughly 2 days as long as the temp is cool. Use it, that stuff is the best!
Use ganache to ice it too, mmmm (sorry, can you tell I'm a chocoholic?)
I guess I am a little apprehensive about refrigerating a fondant covered cake (does the fondant dry out?) but mainly it's just that I don't have the refrigerator space for this size cake!
Yes, I have left it out... and to follow-up on refrigerating fondant covered cakes, they will get tacky. I doing refrigerate fondant, however I rarely use it. I just know it had come out sticky before I had learned that it's okay to leave out. (it dried again though)
Yes, what OCakes said, and also keep in mind your fondant won't dry "hard" it'll just be dry to touch, which is fine. The moisture from the icing will keep the fondant soft, and it will be fine to cut thru and eat.