It really depends on the ferrets and rats if they will bug each other or not. The ferrets could stress the rats out or it could be the other way around. Or both!! When the ferrets run around, if it's anywhere near where the rats are be super careful! The ferrets could bite a nose or tail or anything that sticks out of the cage. Plus they are super intelligent and can figure out how to open cages! Your ferrets may go after the rats all the time and they may not. Just be super, duper careful! Here's some care info, too:
Cage: Ferret Nations are AMAZING!! I love them! A FN 141 will fit 3-4 ferrets. Cost: $140 (check local craigslist for a cheaper one)
Minimum cage size for 1 ferret should be 3 feet by 2 feet by 2 feet. With that though he'll need a bit more extra attention.
Food: Innova Evo, 8 in 1 Ultimate, and Totally Ferret are great ferret kibbles. Cost: $10-25 (also check out raw diets and commercial raw diets)
You can also feed some cat foods like Orijen, Cat Evo, and CA Natural. With food it's best to have at least 3 different kinds to give your ferret variety. Also keeps him healthy!
Litter: Sheets of newspaper, Yesterday's News, and wood pellets (not shavings) work well. Cost: $8-15
Litter box: Don't use the corner litter boxes! Marshall's lock litter box works well. Cost: $8
You can also use cat litter boxes, but make sure he can actually get in.
Bedding: Ferrets love hammocks and should have at least two in the cage! Cost (for 2): $15-20
You can get them custom made from people for cheaper prices.
Treats: Ferret tone is great to have around! Cost: $5
N-Bones are also great! Check the raw food link for these!
Toys: Toys are a must have! Ferrets need a lot of enrichment, so make sure to have a lot of toys around! Cost:$10-20
Just make sure the toys aren't rubber, foam, or latex.
Books: Ferrets for Dummies and Ferrets by Vickie Mckimmey are must haves!
Another thing to think about are vet bills and costs. At least once in a ferrets life (unless you get a ferret from a breeder this may not apply) you'll have to spend $800+ on vet bills all at one time. They also need yearly vet visits. Here's about how much you'll pay a month for one ferret:
Food: $14-$30
Litter: $0-$10
Savings: $15
Misc. (toys, damage, etc.): $5-$100
So in all you'll pay anywhere from $38-$155 a month. Sometimes less, sometimes more. Vet costs are yearly and cost from $80-$150 for regular check ups!
Here are some good websites:
http://forum.ferret.com/http://holisticferret.proboards.com/index.cgihttp://www.hugawoozel.com/ferretcare.htmlHope that helps! Feel free to PM me with any questions!