Would it look bad to stack a 6" onto a 10" if there were gumpast characters on top of the 10" bordering the 6"? I feel like I don't have enough of a ledge on a 10 and an 8.
Well it depends if it is done right, otherwise, a 10" and a 6" is fine.
If you are putting characters on the lower tier, of course. Even if you weren't. That only gives you a two inch border if the upper cake is centered. Not as dramatic as it sounds when everything is decorated. If it was offset, it would give you a nice area for staging a scene or placing several characters.
I think you should be fine. It wont be empty space, is space for decor.