I agree with Keith...(and now Stucco, He just beat me to the post)..Dont restrict your pump.....add a tee to your supply from the pump. Run most of it to the GB and the rest back to the tank. You can make a spray bar from pvc....just drill a seires of holes and let the water spray into the tank.

1470 gph on your pump rating...... That is 'a lot' for your set up.
However, you'll lose 'some' flow depending on the 'head'.... how high the top of the GB is above the surface of the fish water. You'll lose some with the flow of water through the pvc....especially, once you build up a bio-film inside the pipe. ( The bio-film will effect the drains also....always go bigger, then you think you may need, as a safety factor)

On sizing a pump....move the fish tanks water 'twice' an hour....Plus, figure in the things I already mentioned above. (Twice an hour, helps with the oxygen level)
You, yourself mentioned a smaller pump. You 'might' want to try one around 500 gals...just MHO. You really need to look at the flow chart, that shows how the 'head' effects the flow of a given pump.

You can save the big pump for when you add GB's and a sump....remember, it's an addiction....

The hydroton will displace about 60% of the water (if I remember right ?)
Your GB, at 7 inches is about 104 gals...add the hydroton and your GB will hold about '40 some' gals.
So, your GB will fill 'faster'.

Your correct about the fish load....when you don't have as much GB...keep the load on the light side. It's smart, to start with just a few fish....just incase...

Add some red wigglers to your GB....they'll help keep it clean(er). Worms don't drown....only if there is too little oxygen in the water.

Looking forward to your pictures !