No valve on the display drain. You never want to restrict that. You can adjust intake to the sump by choking off the return pump (at the outlet).
Judging by owning multiple skimmers, I would use that turbofloater on nothing larger than a 75 gallon tank.
The size of the skimmer plus the pump gph
is what I'm going by. Look to other manufacturers' skimmers rated for 250 gallons and you will see what I mean.
It's certainly not going to hurt you to use it, but you could purchase something more efficient.
Putting the skimmer in the sump instead of hanging it on the back will also save you in the event that it overflows. If you have an "event" in the tank which causes the skimmer to overflow, it will continue to do so until your sump is empty.
You could burn out your return pump, have no filtration and heat until you are aware of the problem, and have a terrible mess on your floor. There is no good reason to hang it on the back if you have a sump.
As far as rock rubble is concerned, you can simply take a hammer to some larger rocks.