newbie piping questions - what size return - what size overflow?

[Milad]

i really liked bean's design and i like the idea of going with water movers in the tank
The tank already has two holes drilled so can i use those two holes to push water into the tank? and drill three more holes up higher to implment bean's design? all with smaller piping?

here is a drawing of what i mean:
The green pipes are the return and the red are the drain