My old ferret has done well on dry and wet catfood for 5 years. She won't eat anything else, not my doing as I got her late an it's too late to change her. I'm not saying that it's particulary good for them as I prefer raw meat with a biscuit option 24/7. Just that catfood can't be that bad if she's going strong at 5 purely on catfood wet and dry. My other ferrets love raw meat, they smash anything- crows, chicken, bones, fur, absolutely anything.
Forgot to say, give them a treat of a raw egg once every now and then. They love it. Not too much as they can lose hair as too much is bad for t