[hazelrunpack]

Definitely the ferrets need their own space--as Barkingdog pointed out, they're predators and carnivores and will eat, at least, the mice and maybe even the guinea pigs, degus and hamsters.

I don't think I'd try to permanently house any of these species as mix-and-match. Even mice will kill each other if they're overcrowded and, unless you can find concrete research as to what constitutes 'overcrowding' for each species, it might be hard to tell which (if any) combination would work ahead of time. A degu might be perfectly comfortable with another degu in a large cage, but add, say, a hedgehog to the mix and all three animals could be stressed by having strangers in close proximity, even in a large cage. Even if they didn't openly attack each other, they might suffer long term health consequences from the ensuing stress...

If you do some homework and find out that a combination might work, make sure you make any introductions very slowly and cautiously. Just because a pairing might work for someone elsewhere, it doesn't mean that all individuals of the species involved are equally tolerant of each other.

But honestly, though I might allow them all (except maybe the ferrets) to mingle (supervised) in a large space (like a room) during the day, they'd all be sleeping in their own cages at night if they were mine...