live rocks in the sump

[Jedi68]

I have a 77 gal reef tank and I just bought a used in sump protein skimmer. So I am well on my way in putting in an overflow box and a 33 gal sump. Then I am stuck on two questions:

1. Can I put live rock in my sump and how much water should I have in the sump?

2. For whatever reason, if my overflow is plugged, how will I prevent my return pump in my sump from overfilling my main tank?

Thanks...