How big of a mistake did I make?

Hi everyone, yesterday I adopted a 4-month-old male kitten from a shelter to join my one-year-old female cat.

The introduction went much like it does for everyone else—a bit of hissing and growling but nothing too serious. I let them run around apartment to see if they would somehow make friendship on their own. I had managed to do that in the past. But nothing changed, so I separated them for the whole night.
They meowed at the door all night, as if they wanted to be together. I should point out that neither cat was alone, since my boyfriend was with one in one room and I was with the other in a different room.
So I tried to open the door but they would sniff each other and then again, hiss and growl. So I closed the door again.
In the morning my boyfriend went to work so I opened the door again to see if anything had changed. Again sniffing, a bit growling but I could see that they wanted to play together but just didn’t know how?? because their tails were up in a friendly way, not tucked down in fear like the first time.
I gave them an electronic ball and they both chased it but when they got too close to each other, there was again growling.

Also when they were roaming around the apartment, both of them used each other’s litterbox and food bowls so I dont think its about scent or territory. Or am I wrong?

I know about the Jackson Galaxy method but I live in a 1-bedroom apartment and If I would separated them, they would meow very loudly at the door and my neighbors would call the police for animal abuse or sumn. Btw my landlord doesnt’t know about the second kitty so there’s that 😬

Long story short, I’m at work rn and when I was leaving, they chased the ball so I let them together and left. Did I made a mistake? Are they gonna be okay or is there gonna be blood all over the apartment? Is there anyone who did this whole introduction thing this ugly and fast way?