Bottom feeders can't survive just on other fish's poops. They need to be target fed. And they will just poop out the poop they eat from the column-swimming fish. So getting bottom feeders will not solve your poop problem. Living things excrete waste.... that's just the way life is. Getting a darker substrate will hide the poop from you if you don't want to see it. But there is no fish that will eat it and make it go away.
To answer your question about overfeeding/underfeeding, I think you're doing just fine. When you see sunken stomachs or swollen stomachs, then you know you need to adjust your feeding schedule.