Introducing new 5 month old kitten to 3 year old resident cat

I adopted a male cat (Leo) from outdoors last winter who was about 1.5 years old. Social, sweet, and very fiery/opinionated lol. I used to take him on harness walks at least twice a week, and we lived in a bigger house with space to run. We moved into an apartment this January and since then he has become a bit more sedentary, sleeps a bit more, gained a bit of weight. We go on harness walks sometimes in the hallways or I’ll drive him to a park, but it’s pretty rare nowadays (living in the city on the 6th floor…). So, we decided let’s get him a kitten to hopefully get him a bit more active and have something to do when my boyfriend and I are at work. Found the perfect kitten, and 5 days ago we began a “trial adoption” of 2 weeks. We have been following Jackson Galaxy’s guide pretty much to a T. It has been going as expected, nothing too crazy, but I am just nervous to take the next step of putting them in the same room.

Basically, we have set up base camps, let them explore each other’s areas, scent swapped. They did not see each other in any capacity for the first 4 days. Today was the first time we “lifted the veil” a bit. Basically through a screen door, we had a blanket covering the view while the ate breakfast. We lifted the blanket a tiny bit so they could see each other. Leo (as expected) hissed and growled a bit. But he made no sudden movements, and he did continue to eat afterwards and he accepted pets from me.

I’m looking for words of wisdom, advice, suggestions, etc. I’m just nervous to keep going because I don’t want Leo to hurt the kitten if he decides to attack, and I don’t want to scare the kitten, or make either cat so uncomfortable that they don’t trust me anymore.