Is it true that ferrets don't get along with other pets?
The answer is a qualified "no".
I had two ferrets. The oldest was always, always happy to make new friends, regardless of the species. He was very excited to play with any new dog or cat he met, and would do the ferret "bounce" that typically indicates play.
My other ferret was more curious than playful when it came to other species, but I wouldn't say she didn't get along with the other animals. It was more that she was simply disinterested in them as play companions (she was very attached to any other ferret she met, though).
However... every cat that saw my ferrets immediately freaked out. The moment they caught scent of the ferrets, they were immediately on the alert, and when they saw my ferrets, the responses were invariably flight, or hunting/stalking/aggression.
My friend's golden retriever was fascinated with the ferrets, and would gently poke at them with his paws, while my one ferret bounced back and forth in play.
Whether the ferrets and the other pets get along is largely a factor of the temperament of each individual animal, as well as the circumstance in which they met.
I do know some people who have had both cats and ferrets living in peace together in a household, so they are certainly capable of learning to like each other. I would suggest that the same steps for introducing new cats to each other be used for introducing ferrets to cats. Go very slow, and take a bit more time with each step than you would with two cats. Be aware, also, that ferret play typically consists of behavior that most cats will initially confuse with aggression: lunging with an open mouth, nipping, and attempting to leap onto the playmate's back for a nip at the scruff of the neck.
The last one is particularly problematic, as ferrets have rather sharp teeth, but very thick, loose skin, so a ferret can grab another ferret by the scruff quite hard without causing serious discomfort, but a cat will most certainly not appreciate it.
I would also be very cautious about introducing a ferret to other species of common pets.
Other "pocket pets" such as mice, hamsters, gerbils, etc., birds, or reptiles may actually trigger hunting instincts in a ferret, and the resulting interaction may result in serious injury or even death for the smaller creatures.
Similarly, kittens or puppies would be at risk from a ferret that doesn't know how delicate they are.