I know that you can have 5 GPUs on a motherboard for mining, though I know absolutely nothing technical about how that works. Is it possible to run 4 cards in CrossfireX/SLI mode for gaming with the 5th one idle, and then leave the 4 Cards in CFX/SLI with the 5th one separate for mining?
Yes you can do this. 4 cards is just the maximum for gaming, you can have as many as you want for compute purposes. If you dedicate any amount of cards to a xfire setup you can use the rest for whatever you want, in this case mining.