Possums are darling until they open their mouths--they have the most teeth of any north american mammal as I recall
We've never had any problems with possums, but when cornered, they can put up a good fight. So yes, there's a chance they might have a run-in with a cat and hurt it, but would a cat attack something that big in the first place?
There's always the possibility of diseases and worms being passed from wildlife to pets, but getting rid of one possum would likely not stop that from happening--there's always some other critter to step into its place. So you might want to increase the deworming schedule on your pets, or at least take in more frequent fecal samples for testing.