First, you do not need GPU's to be running in Crossfire mode for the purpose of mining, just being present on your computer should be enough.
If you are wondering about Crossfire for those times when you aren't mining and want to run today's equivalent of Crysis (no clue what that might be) at max settings, then from my understanding Crossfire works between any GPUs as long as they are from the same generation, i.e. all cards hd 6000, or hd 5000.
The most cards I have personal experience with in Crossfire mode is 2 GPUs. I do not know for sure what the case would be for a 6990 plus another card, as the 6990 is already two GPU's in and of itself. I know you can Crossfire 3 cards together, but I have no idea about a 2 cards with a total of 3 GPUs.
I have heard of people crossfiring two 6990s together, so that leads me to think you can crossfire a 6990 with a single GPU card. Do some google searches, I bet this has come up before.
Three 6970 cores Crossfired together?
What are you planning on doing with that power? What game or other app needs that much? Today, anyway.