My new ferret isn't getting along with my other ferrets - help!
When you bring a new ferret into your home, it is natural to see some friction between the new ferret and your other ferrets. It's going to be difficult for all of the ferrets - the new ferret has a whole new environment to get used to, and your other ferrets have to deal with the stress of a new animal in their territory. So it's completely normal for your ferrets to be suspicious and even hostile of a newbie.
The best way to introduce a new ferret into your existing group is slowly! First, you want to quarantine the new ferret for a minimum of 2 weeks. Before you introduce the new ferret, make a vet appointment to have him checked out. Once the new ferret has a clean bill of health, start slowly. Try to introduce the ferrets in a neutral area. If you already have multiple ferrets, introduce them to the new ferret one at a time to reduce stress for the new ferret.
Do not just put the new ferret in the main cage. Use a separate cage for the new ferret, and put the two cages next to each other so they can get used to seeing each other. You can also switch bedding back and forth between the two cages so all of the ferrets can get used to each other's scents in the cage.
Supervise playtimes initially, but don't interfere unless the play gets too rough. There will be some typical rough play, but signs that it's getting too rough include fear pooping, screaming, and biting and shaking roughly.
To find more information about introducing a new fuzzy into your home, check out the following article. It's full of helpful information!

