corys and substrate

Hello,

My newly/barely cycled 50-gallon bowfront has 2 dwarf Flame gouramis in
it. Time for some more fish! Here are some photos of my tank:

I would like to add either 4 Rainbowfish or 6 Corys next (not both
groups at once, though). Which would be the better choice for my next
addition? (BTW, I plan on a maintenance schedule of gravel vacuuming
and water changes of 20% weekly, which I believe is the standard
recommended amount, yes?)

Other question: I have round, smooth "Wonderock" gravel in neutral,
varied browns, beiges, blacks, etc. The stones are smooth and fairly
large - maybe 1/2 inch or 3/4 inch on average. Not the typical tiny
gravel. Is this substrate OK for the corys? I have read that they
prefer fine river sand. I don't want to hurt them! But the gravel is
definitely smooth and rounded, not rough . . . It's just larger
pieces.

Oh, and my water parameters are holding steady at 0-0-0. My pH is 7.2
and the water is soft.

Thanks for any advice!

- Lisa