Any ideas will be welcome! Thanks

There is medication for the bully cat to help him relax, plus Feliway. I'd also
try to play with the bully a lot to tire him out. Then I'd let the bully smell
the blankie of the new cat to get used to his smell. I'd brush them both with
the same brush. Also try to reward bully cat whenever he is near the new cat
and being nice. In your limited space, try to make an area just for the bully
cat, and one for the other cat. Give them each their own scratchy tree, pillow
and litter box, so maybe they will feel like they have their own territories.
Worse comes to worse, it might be an idea to start looking around for a good
home for the new cat. I have to place one of my nice cats because my bully cat
was not to nice to him. I waited til I found a great new place for him.