I have the main drain right at the bulkhead, and I use a large strainer over it, (I actually cut out every other piece of plastic in the strainer so small particles are less likely to clog it. This is just a preventative measure against small fish/snails getting stuck in the gatevalve.)
The backup drain should be about 1" below where you think the tank water level will be. The further lower it is, the more splashing you will hear when water goes into the overflow box, the further higher it is, the less likely your overflow box will skim the surface water out.
Also, the center drain for your frag tank will likely have a slow leak to the floor unless the durso is glued to the bulkhead (which I don't recommend, durso should never be glued IMO). Instead, glue a ball valve under the bulkhead and turn it to the closed position.
Only last thing I can think of is, you might want to consider having more unions. If you ever decide to move certain plumbing around, you'll appreciate the fact you added unions for easy maintenance. I would add unions after each drain for starters.
