You've got a number of fish options in a 20gal reef. 20 longs actually can be better for offering more territory than the high equivalent, but I'd still stick to a small number of fish since the bioload works differently for marine than for freshwater. For, say, 4 fish you could pick from a variety of:
- oscellaris clowns (easy to pair, good beginner species usually as long as you don't get super-facy ones)
- cardinals (can be kept singly and mixed; caution trying to do a larger group in a nano)
- basslets (a little less hardy)
- bottom-dwelling / grazing blennies (kept singly)
- bottom-dwelling / sand-sifting gobies (kept singly) - requires reasonable sand depth
- firefish only if you have a REALLY TIGHT FITTING lid on the tank with mesh or something around all opening around filter tubes and the like; they are ninjas for jumping. While they do get kept in shallow tanks, from personal experience I can tell you that the risk of having a fish go flying over your shoulder when you open the lid is non-trivial.
Sometimes you can get away with a single small damsel species and 1-2 other very different looking fish (but no clowns - too similar). However, many damsels are quite aggressive, so even though they're abundant in the trade, very hardy, and cheap, they aren't the best fish for a first nano. I'm personally a fan of damsels because they're very inquisitive and plucky, but I definitely wouldn't recommend them for a first try at a community nano.
For cardinals and basslets, if you don't get an existing pair, a pairing is not guaranteed to work and you may need to be prepared to return one if it goes badly. Cardinals in particular don't often like to be in groups larger than a breeding pair once they hit adulthood. Basslets I've heard are a little less likely to be a problem if you end up with the wrong genders (or just a super stubborn fish), but they are also more sensitive to tank problems and stress. You can also typically mix cardinal species with single individuals and achieve a happy group. For example, I've several times kept a single bangai with a single PJ and they hung out together quite happily without having the breeding behavior-induced aggression of same-species pairs/groups.
With any of the fish above, you can can pick fish by roll, like 2-3 free-swimming fish and a more bottom-dwelling blenny or goby.
On the coral side you've also got a lot of options, largely depending on the lighting you get. The nice thing about a 20 long is that it's shallow, which means light penetrates the whole depth of the tank better than for taller tanks.