you definitely can keep them on the sandbed.....it depends on the ccoral and the light. you should light acclimate from the sand and move up anyway.... though, with a360 you can control % of intensity so you theorhetically dont need to acclimate corals in the traditional way.....
if you are still getting used to your lights, take it slow, and remember that the leds look less bright than they actually are....hence people frying corals.
I am also very curious about a kessil 360 on a 20 L tank....its gotta be a superfluous amount of light. what are they set at