If you shoot for the smaller spotted silver dollars, they won't be the big, bulky, bio-load increasing problem you're thinking of. I have 4 of the smaller ones - they're normally spotted even at a small size and look different than the solid silver ones, so you should be able to pick them out of a group.
They're either Metynnis maculatus or Metynnis lippincottianus, I forget which is which. Either way, if you Google image search either, you'll see what I mean. They stay like 4-4.5" and are very quick swimmers, so they won't get eaten unless you buy them way too small. Plus they don't get 7+" and eat you out of house and home like the others.