.exr file quality options for seperate renders? When rendering out a scene where all render passes cannot be placed into one .exr file, what is the best alternative for additional passes to be at the same quality level as an .exr? 

For example:
If you are rendering a scene and can render diffuse, ... etc (all the usual passes) in one .exr file but later learn you need an additional pass or two which would be added to the same composition as the .exr file, what file type would use? Would another .exr be the most efficient choice to keep the same quality even if it only has one or two additional passes for example?
Yes, another EXR would be the best option. Any other file type will have a hard time matching the quality and compression method. And it's not as if the EXR will be "bloated" with lots of passes without data. It just won't store data that you don't give it.

It'd also be more convenient to use the same types for the same things. If you're going to be pulling in passes for compositing, it makes sense to use the same type of file for that same process, even though it might contain different passes.