So my housemate Jesse and I lost a housemate back to her family who missed having her home XD

We have lined up a new housemate....who's bringing a new cat.

Now the original introduction between Jesse's 14 year old cat and my puppy (6 months at the time) Was pretty simple. We worked hard on OB, let the cat investigate the puppy through a crate for a few days until they pretty much ignored each other. Banned the puppy from Jesse's bedroom which remains a dog free cat area. When the actual first contact came around Ryuk was on a leash, the cat (who's raised 2 puppies in his time and is dog friendly) walked up and headbutted him in the face. Ryuk, who'd been getting used the cat at the same time while in the crate responded by licking his head. They've had their drama (Ryuk wants to play, cat does not) but there is no aggression between them and they get along wonderfully. Ryuk is crated when I'm not around so they're never left out together but I'm confident it wouldn't be an issue at this point at all.

However this new cat (who's also older at about 12 years old) has lived with dogs but runs from them terrified (the dog were never properly introduced to the cats in the original home and have been allowed to "play" with the cats). Our current cat has no fear of dogs, he doesn't run. Ryuk will chase cats that run (he thinks ti's a fame) but he can be called of with no problem and he won't hurt cats when he catches up to them (we petsit for a 2 year old 5lb cornish rex who loves nothing more than to play tag with my dog) but I'd rather not scare the fur off of this shy kitty.

There are three bed rooms in this apartment. Two built in (one mine, one Jesse's which is her cat's room) then one we built with a solid temporary panel board and 2X4. It currently does not have a door (as our previous housemate prefered a curtain), but has a low step over barrier. Ryuk has never been allowed into the room, our current cat has.

I've done kitten dog intros, cat puppy intros, cat cat intros, cat kitten, dog puppy, dog dog....but never cat dog cat and never with two cats this age and a high drive dog and no space at all. Any advice?

Cat to Cat is honestly the one I might be more worried about, you can train the dog to behave around the cat, but training the cats to be nice to each other is a whole other can of worms!!

I would make sure Ryuk has zero chance to chase her, and give her time to warm up to him.

Could you put a taller baby gate up that the cat can easily jump but not Ryuk?

Or one of those tall cat climbing things so there is somewhere up high in the common area for kittty to get away?

Everything Jennifer said plus more:

Cats don't handle stress very well and could develop health problems, such as Feline Idiopathic Cystitis , for example.

I would keep the new cat completely separated from both resident cat and the dog. Behind a real door at first at least.

If sometime down the road the newcomer exhibits the desire to be friends, then you might consider some kind of introduction. Right now the stress of moving would be enough for the cat to deal with.

With this cat (our current resident), I'm not as concerned about that cat/cat intro as both cats have been in homes where foster cats are in and out and both cats have proven up to this point they are completely indifferent to other cats. They will each have their own space and will not be interacting with each other for some time.

Clarence (current resident kitty) is perfectly happy staying in his room (he's got everything he ever needs or wants in there). Ryuk can't be confined to ONLY my room however (there just isn't enough room, I have the smallest bedroom by far, his bowls along take up about 20% of the other space I have). New Kitty will have living room and housemate room access until the proper door is in place (we're still looking for one that fits...we intentionally build the doorframe standard size but so far our dumper raiding efforts have produced only undersized closet doors). Ryuk is allowed in the living room when I'm home, so New kitty will be most likely meeting him first. We do have the room barrier up (kitty goes over, Ryuk is NOT allowed to jump the barrier and follows the rules well).

I'll be making sure that I'm ALWAYS a step away with the dog around new kitty, but I don't see as much issue for a cat to cat intro in this particular case.

That is good that the cats are neutral, it is never fun when they don't get along or one harasses the other one.

I think your plan sounds good. I might even consider tethering Ryuk the first couple of times just to make sure he can't run at her, or do anything scary even though he doesn't want to hurt her.

make sure new cat doesn't have fleas. cat fleas are the worst IMO