i would go with a mag 9.5 and gate valve to throttle it down if you dont like that much flow. Cuz i know 1" pvc can handle 600gph, and as long as you have the return pointing down the long part of the tank, it shouldnt cause a sandstorm, and you can even split it with locline if you wanted. for the drain i owuld go with 1.5" even 2" if your combining it, i would rather have more than enough room if i needed, rather than having just big enough pipe.
My sump is 14" tall, and i have the baffles 2" below the top,for you i would say 4" below the top, cuz with that much flow, your gonna have alot draining down when your return turns off. my .02