I'm curious, with everything you're changing and adding in, how big is the final rom going to be? Are you using the same mapper or changing to something like MMC5?
If you ask me now, the answer is i am working on the MMC3 Mapper resulting in a 728kB ROM. Upgrade to MMC5 is not planned, cause Editroid doesn't support it.
So yes, space can be a problem.
Everything has to be planned carefully and space has to be used efficiently.
For example i constricted all the parallax frames of the whole game to one part of the chr sheets. Parallax scrolling is using 32 frames, every frame is using up a quarter chr sheet) it is implemented this way).
There are 64 chr sheets, and parallax scrolling in my hack alone uses up 8 of them.
So much to the maths. ;]