Resident cat downstairs, new kitten/cat upstairs — how to introduce?

My adult cat Rupert (3 y.o) currently lives with me in a downstairs studio apartment within my mom’s house. It’s a long, complicated story (I was worried I might move out which isn’t going to happen now for the foreseeable future, and also he eats things he shouldn’t sometimes so my apartment is very cat proofed) but we want to now try to give him access to the whole house.

In addition, we really want to get an additional cat or kitten. Rupert was originally from a cat cafe and to my knowledge, he does very well with other cats.

What is the best move here - I am thinking we keep Rupert in the apartment while the new cat gets used to the rest of the house, and when new cat is settled, we do swaps in spaces? And slowly introduce each other’s scents? Or should we show Rupert the bigger house first before getting a new cat? We were slightly worried that introducing a new big space and a new cat would be too much going on for Rupert - could he get territorial over the new big space? Any advice would be super helpful, thanks!