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