yeah for a small 20G you could get away with that, an Aquaclear HOB
like a AC50 would be good.
for me personally (i tend to max/over-stock) all those fish would be fine, except the scooter blenny, but for a different reason than stocking. scooter blennies (like mandarin gobies/dragonettes) feed on copepods, a small animal in your aquarium, problem is, they constantly eat and could wipe out a 20G in weeks and die of starvation unless you supplement pods
(expensive) or have it eat prepared foods(training i wouldnt reccomend to a beginner, and buying one that eats it doesnt always work)
dry rock you could throw in as long as you leave it in some SW
(seperate, like a bucket) for a while and make sure it doesnt cause any PH swing or an ammonia spike.
as for lighting i'd listen to what was said above, i'm not too keen on lighting