Aquarium Rocks

I'd use aquarium silicone. You'll have to let it cure for a few days before it goes into the tank, so be patient! My only concern about building this rock wall is that it might make cleaning difficult, but it sounds like a cool plan.

*edit* I've seen polished river stones at Walmart. They sell them in bags and they are pretty cheap I think- that should work.

Not glue. Use Aquarium safe Acrylic. You can find it at home depot or lowes...and it's less expensive there than at petco or petsmart.

A turkey baster is good for cleaning between rocks without having to move them, just blast the dirt out

There may be a risk in having gravel too deep behind the wall. you'd have to wait for someone more knowledgable than I on that one.
Similarly with the "what rocks" question.
The answer is, "Any clean, inert rock of approptiate size without sharp edges."
big help, huh?
I know that quartz, granite, or slate would work. I have never had trouble with any rock I have ever put in a fish tank. rock,(pebbles) from a nearby non polluted river or creek bed should be safe. I assume that anything you buy from a fish store would be safe. They also sell "river rock" in various sizes at landscape places. I bet theyd give you a handfull.
I say find rock you like and try to learn what it is. Posting a picture here would help.

what glue?
Definitely, Aquarium Safe silicone. Check the DIY forum for good places to buy that.
iIhope all that helped a little