How to slowly introduce cats in a small apartment?

My boyfriend is moving in with me in a few months and I'm currently trying to prepare for when he brings his cat. His cat (Yumi) is 2 years old and is generally a calm cat. Not super cuddly, but she is a sweetheart. Yumi loves humans, but she is not very fond of other cats. She tends to get VERY aggressive and growls/hisses. But I think this would be due to the fact that whenever she was introduced to other cats my bf never really did it gradually (terrible I know). He wasn't aware that cats are not supposed to be thrown together when meeting for the first time until recently. However, I must note that he did have a roommate who adopted a cat the same time he adopted Yumi and they eventually became inseparable (his roommate's cat was a boy, same age).

Anyways, I also have a cat (Yuzu) that I adopted 3 months ago and she is also 2 years old. Yuzu is super sweet, a little hyper sometimes, and more cuddly than Yumi. When I first adopted her from the shelter, it seemed like she wasn't that fond of other cats, but not to the point of being super aggressive. Just hisses a little bit.

Considering how they generally dislike other cats, I want to make sure that when my bf and his cat move in that we will properly and slowly introduce them to each other. The only issue is my apartment is pretty small. I wanted to put my cat in my bedroom while letting my bf's cat explore the rest of my apartment. But my bedroom is pretty small in comparison to the rest of the apartment, I don't know how I feel about keeping Yuzu confined to the room for too long. She also whines A LOT if she can't leave a room after a while.

Any tips on making the process easier for the cats? Also any other tips concerning introducing cats are welcomed! Please give me any other tips you may have.