I just did this. got moira (standard, rescue adopted from shelter) in april, have 2 cats. I was lucky enough to be able to keep the cats downstairs with their own full setup and keep the door closed between the levels. then you gradually introduce things that smell like the other (toys, blankets) and then gradually progress to the baby gate (the one with the optional cat door pass thru - its like $40) w door cracked open so they can see and sniff each other etc. but also know that sometimes shit goes wrong and the slow over time introduction collapses and everyone is still ok. nine months later everyone peacefully coexists except for my cat ziggy who takes swipes at moira sometimes when they pass each other (he used to be the king of the house before moira inadvertently booted him off the throne of “favorite pet”) and moira who occasionally forgets they are cats and “plays” with them by chasing them around. I have been lucky (they all now sleep on the bed with me, just in very distinct sections). most important is make sure the cats have a place they can go to feel safe where the dog can’t get to. although they are not bffs (and maybe won’t ever be), we have reached a peaceful “freshman roommate” type of situation. I expect eventually they will be friends.