Cats Fighting or Playing?

The growling and the hissing is probably because they are not used to eachother yet. This is perfectly normal. Cats are very territorial at first but give them a while and they will be cuddling while they sleep. Maybe try putting the cats by eachother but not close enough to let them attack one another. Pet both of them so they know the other one is not taking their place. It might take a few days to a few weeks but they will eventually get along. Hope this helps