If you don't want your pump in the FT you can get a portable utility pump and mount it outside of the tank.

I think you're going to find out that water being pump will flow so much faster than water just running off the top of your FT though your 5 gal filter on the way to your GB and back to you square sump.

I'm sure that there is some formula that could give you that answer. You might try just bypassing your 5 gal filter and your GB, just go from the slump to the FT than back to the sump. Once you find the right size on the water return pipe so everything works. Then plumb the rest of your system, just to be on the safe side I would use the next size large pipe (it wouldn't hurt).

If you do end up with an extra hole coming out of your FT. I would use it as an emergence overflow to the sump, it will stop your FT. from over flowing and stop your pump from running out of water.

Hey try this first, use one of your old buckets as a pretend FT. Pump water into the bucket...cut hole on side of bucket to find out what size of hole is needed to return water back to pump.