If the sump is on the floor, and the top of the tank is at five feet, then your head pressure is about five feet. At that head, with your 30gal display, the sicce 1.5 would be returning about 180gph and is good.
A manifold s a good way to cut down on the amount f pumps you have, and a valve on each line a good way to control flow, but how much flow becomes a guess. You could time and calculate each line at the appropriate head height to get an estimate, but each time you adjust one line's flow it effects the gph on the other lines. I recommend you keep your return line on a separate pump.
When it comes to a return pump bigger is not better. Have seen many tanks with minor long term ailing problems settle down nicely, simply by getting the return down to 4-6x the display.