Other non-Nanite geometry in the scene can take up frame and won’t scale with number of screen pixels like Nanite does but other than taking up frame time they won’t have any other negative interaction with Nanite. Non-Nanite pixels will occlude Nanite.
The primary reason to use lower resolution Nanite meshes is to save on disk space. The performance of lower poly meshes may not be intuitive or like you are used to. Commonly that won’t improve perf. Sometimes it can even be slower. YMMV.