The number and type of fish you can put in a 10 gallon is pretty limited, a 5.5 even more so.
I was going to recommend that you check out the Small Aquatic Garden contest entries for the AGA aquascaping contests, but the stock in a lot of those tanks is based on 1) Insanely dense planting, 2) Frequent 50% water changes, 3) Massive filtration, and 4) Wishful thinking. You can get ideas there, but, as a f'rinstance, I saw a half dozen tanks there under 10 gallons with a Siamese Algae eater in them. Now, the fish in the picture was an inch and a half long, but they get 6" pretty quickly (mine are 6 months old and pushing 5"). No way should they be in any tank smaller than 50 gallons.
So guide on the following:
1) Labyrinth fish with a maximum size of 3" or less. Bettas (one male or 6 female), Dwarf Honey Gouramis, Pygmy Croaking Gouramis, Sparkling Gouramis
2) Slender (Minnow shaped) schooling fish of up to 1 1/4 inch: up to 2" of fish per gallon, 3" per gallon if you plant the devil out of the tank and do frequent water changes. Neon, Green Neon, Ember, Glow Light, Blue, and Gold tetras, Microrasboras and the three small rasboras - Trigonostigma heteromorpha (Harlequin), hengeli, and espi, Endlers Livebearers, White Clouds
3) Deeper Bodied and larger schooling fish not noted for high activity (danios are right out!) up to 2.5" long. 1" per gallon. twice as many in a tank that's been heavily planted for more than 6 months. Emperor, Flame, Pristella, and Black Tetras, Cherry Barbs, Platies, Guppies
4) No more than 1 otto per 3 gallons. No more than 1 Pitbull Pleco in 5 Gallons
5) Up to 1 Dwarf Frog per 5 gallons (will eat those listed in #2)
6) Up to 1 Mystery Snail per 2.5 gallons
7) Up to 4 Neocardinia Shrimp, or 2 Amano shrimp per gallon
8) One female dwarf puffer per 3 gallons, one male per 6 gallons (Male and female territories can overlap a little).
9) Up to 2 bumblebee gobies per 3 gallons (Brackish tank)
10) One male and 2 female White Cheeked Gobies (Rhinogobius duospilus, formerly wui)
11) If you have room for no fewer than 6, you can get up to 2 Corydoras pygmeus, hasbrosus, or hastatus per gallon, or two C. guapore per 3 gallons, or 1 C. Panda per gallon. Do not get fewer than 6.
Keep it small, keep it peaceful or a species tank, and ask before you buy. Remember that fish will not stop growing just because the tank is too small. Also remember than, you go to the bathroom to go to the bathroom. Fish live in their bathroom. I think that's what did in your goldfish and cichlids.