That's not a lot. Try something like 3k3 on 3.3V and 4k7 on 5V.
Can you share a schematic and pcb design?
Depends; see question above. The reality is that you're taking us into that direction and in my experience, what you put in is what you get out. So if you make this about pullups, then people will talk about that.
If your concern is that your I2C ceases to work as you add another device to it, then show us what the project is about, how you've implemented, what the working situation is and what changes exactly occur that make it stop working.