It is confusing because there is not always one correct way to go. FOWLR
are much more forgiving than reefs though. I think that a skimmer is the better way to go, but a hob
will work. Filters will need to be cleaned regularly as the build-up of waste will increase nitrates. IMO
, skimmers are a little easier.
One power head will be fine if you can get it so that there are no dead spots. Dead spots allow waste to build up and lead to nitrates.
Clowns are very great as a starter. You will want to get Ocellaris Clowns as they are going to be okay in a 20g. Others are better in a 30g or larger. They sell tank-bred too, which is kind of cool in that they should not have any parasites/disease and they were not taken from the wild. After damsels (which are mean) they are probably the next best for being hearty. In a 20g, you are not going to be able to add two many more fish. In case you did not know, clowns do not need anemones to be happy, but its a cool relationship. You just want to add anemones early in the game, as they want a very established aquarium to thrive.