You're quite limited for appropriate stock.
www.nano-reef.com might be a good place to skim to see, lots of fusion 20's there.
When I had my fusion 20 running it was stocked with:
2x ocellaris clowns
1x golden yellow assessor
1x pinkstreak wrasse
1x tailspot blenny
The clowns long-term can create problems if they start being hosted, especially if they're mating. Other small nano appropriate fish can include:
Springer's damsel
Possum wrasse
Multitude of nano gobies (neon / red-head / greenbanded, shrimp goby, etc)