Yeah you want them bare bottom. I use large PVC fittings like 4" elbows and T fittings to give the fish a place to hide. Since they're plastic I just rinse them and dry them.
I use the 10 gallons for a lot of fish. For larger fish I will go with 20 longs, but since they're only in those tanks for a short period of time, it's not usually an issue.
I also cover the tops with some old sheets of Plexiglas to help moderate evaporation and stop jumpers.