Yes, gravel is fine for a drainage layer. Don't forget the screen material between that and your substrate. You can use a less than 5 dollar roll of window screen material over the pet store substrate mesh and save money too.

You only need about 1 to 1 1/2 inch deep drainage layer

My first experience with hydroballs:
I have a Exo 18x18x18 and i thought i needed to cover the bottom with bags of hydroballs when purchasing the whole setup. 4 bags looked like 2 inches at the time. I got home dumped 4 bags in, added substrate and wet it all down for get a pump going for a water feature. next thing I know, I have about 5 inches of hydroballs in the tank :-) Substrate was floating above them since the clay balls separated and began to float :-) This was because I needed to get the water level above the small pump. What I mess I had.