I wish you the best of luck and lots of patience

We successfully introduced an energetic spaniel puppy to our home where our aging cat has been used to ruling the roost for thirteen years. The advice I followed, which seems to have worked, was to leave them work it out between themselves, each learning to respect the other's space. In practical terms this meant not intervening when the pup tried stealing from the cat's food bowl, leaving them to sort out their arguments (pup teased the cat, cat retaliated by showing pup what claws are used for), leaving one muscle in on the other's territory, etc. We weren't cruel, however, and gave the cat lots of time to be on his own in the house too, in the early weeks. Pup was initially confined to the garage, and little by little let stay for longer periods in the rest of the house while we were there. Nowadays (6 months on) the two sleep together in the garage, and each still loves eating from the other's bowl!
With four cats, however, I suspect this may be slightly more difficult with one.
I hope you can find a way to make it work.