Ferrets are very social animals, so keeping at least two ferrets together as a pair rather than a single pet is recommended. But housing ferrets with other species is very different and requires careful consideration.
Ferrets are strict carnivores. In the wild they prey upon and eat whole animals including small rodents. Thus we would not recommend mixing an adult ferret with any other small animal species as this may present too high a risk to the safety of the small animal. There are cases where ferrets have hunted and killed kittens and other small animals such as hamsters in the household. This is likely a result of their natural instinct to hunt small animals for food.
Mixing an adult ferret with an adult animal of similar size may also be hazardous, though the risks are likely to be less than with smaller animals. Introducing a ferret to another animal species must be done gradually and any interactions between the animals must be supervised, not only during the introductory phase, but also on any occasion when the animals are mixed together.