We had a dwarf freshwater puffer fish in a 5 gallon, with 1 bristlenose pleco (they stay smaller than other plecos).
I've also had a beta in a 5, or 1 goldfish - but they will need more space as they grow as goldfish can reach up to 8 inches in length.
Could also do some tetra's..
The basic rule is 1 inch of fish per 1US Gallon. But you want them to have room too, so if you did 3 tetras you'd have 2 gallons "extra" for them to hum-dee-dum in. Or a beta, and you'd have 3 gallons "extra".
Currently, I have a 55G tank with 2 black moors, 1 lion head, 1 calico, 1 comet (my son won it) 1 female beta, 2 frogs, and 3 bristlenose plecos (2 female, 1 male - my previous pair bred and I had like 100+ to sell, the 1 female is Mom, other is daughter and male is son - they're being moved to a 30 once I get that tank going). We're also getting a celestial for the kids for Christmas.
And once everyone grows a bit, we'll have to get a bigger tank - haha!