Hey,
The drain type you're looking for would be an "external durso". The idea of the T off the back of the tank, with an air vent is to create a tunnel of air down the center of the pipe, so the water flows quietly down the pipe; this won't be silent, but quiet.
If you're not running the T or air-flow into the pipe, and just a 90, then you would be trying to create a syphon (herbie) which you cannot do with that type of overflow.
Do you have plans for a backup drain??
I cannot find the proper picture of the bottom drain for in-sump that I was thinking of... but this is something similar: (found on nano-reef)
http://www.daemonfly.com/images/2.5g/lowerdurso.jpg