You really don't want to put any rocks or pebbles because if the cham eats some dirt it will normally pass. If it on accident eats a pebble or a hydro ball that could result in death by choking or not being able to pass it (poop it out).
Also larger rocks can be dangerous as they can fall if cage were to be knocked over, or if it was a female digging then the rocks could collapse into her hole.
Also bugs will do their best to hide in anything such as the crevices or large rocks then the bugs will wake up during the night and bug your cham.
One last option would be the repti-carpet, it may get pooped on and could harbor bacteria but as long as you sanitize it often that would probably be a smart and safe option.
I use large river rocks and have never had any problems. The crickets can hide under them, this is true. Hydroballs definitely not. Those could be eaten and causing death by choking or not being able to digest. The reason most of us cover the soil is because the chameleons seem to sometimes like to eat the dirt. If you don't have a female then you really don't have to worry about her trying to dig up the pot to lay eggs.