you don't need separate versions for perspective and orthographic cameras
no ortho cameras required here. not sure when one might want both. perspective is generally considered superior to ortho for most things, so if you use perspective, there's little point in dropping back to ortho for some things, it just makes you ortho graphics look worse compared to your perspective graphics. that's why a uniform level of graphics throughout a title is desirable. you don't want the good stuff making the bad stuff look even worse.
I'm not sure what you mean by number 2. sounds as if you want to scale individual objects? That would lead to all sorts of issues (for example objects starting to interpenetrate after scaling).
yes - i suspect that would be likely at extreme magnification levels.