actually 6 or 7 is to many for a 300... id i wouldnt do more than 4 in a 300. may seem big now, but they do get to 12 to 14 sometimes up to 16 inches... but nice tank regardless.
With 6 O's the best bottom feeder IMO would be your Python.
I get that you think waste will be an issue. I have an ammonia sensor on my tank, and excellent filtration. I perform VERY regular water changes and tests. I have power heads positioned so waste goes straight to a filter sock instead of settling on the bottom. I know what's required to keep the water clean, and it stays clean.
With 6 O's the best bottom feeder IMO would be your Python.