As 20 longs aren't that deep, esp with a DSB, I think the eggcrate covering would be a good idea, too.
These are fish that I would like in a small tank, in no particular order:
Ocellaris or Percula Clown; Lawnmower Blenny ( Salarius fasciatus); Midas Blenny (Ecsenius midas);
BiColor Blenny (Ecsencius BiColor); one of any of the clown gobies (Gobiodon sp.); Highfin Banded Goby (Stonigobiops nematodes)-actually, just about any of the gobies other than those in the Valencienna family, they are
sand sifters and really like to dig, too!
A Royal Gramma could certainly hold its own against a small damsel; maybe a Yellow Tail, A Fiji Devil (Chrysiptera taupou) or the Austrailian counterpart to the Fiji which has orange with the blue, rather than yellow, it's also a Chrysiptera sp.
So many to choose from!
Alice
One more fish the size of your gramma would be safe, but I would hesistate to go beyond that. To bad on the clown, that would have been my next suggestion. or a couple small blenny or goby types(no mandarin)
"A BRW Original"
