i've had 'finkids', lol, for 6 years now. Started with a betta and a little half gallon fish bowl, that quickly turned into having 6 bettas in varying bowl sizes and 10 gallon tanks, then my first 20 gal, then my first 55, then my second 20 gal, then my second 55, then my third 20 gallon....... lol. It's been quite the fishy adventure over the years for me. And now I have a 55 gallon with my two big fish in it and a 20 gallon with a couple live plants and a few tropical 'community' fish. Just freshwater though. Not ready to take the plunge to the dark side (saltwater), lol, and probably won't. Saltwater tanks are a lot more work than freshwater, and the smaller the tank the more work it is to keep the water parameters stable, whether it be fresh or salt. It's enough for me to keep my two tanks as nice as I can, so definitely don't need the added work and stress of salt!
My suggestion would be to go with a tank a bit bigger than 5.5 gallons, in order to enjoy your tank better and not have to be working on it all the time. 20 gallons or more would be good. That way if your water parameters do fluctuate, you have more water volume to 'absorb' that fluctuation and your tank doesn't go downhill so fast or 'crash' as us fishkeepers call it. A '20 long' would make for a nice little reef tank and due to the size, you could keep a nice little assortment of fish and inverts in there. A pair of clownfish and peppermint shrimp would top my list for inhabitants of a saltie tank!