i personally wouldn't put anything with a betta in anything less than 10 gallon, gives them room for there individual territories
and your tank would be overstocked even if you had the dwarf kind.
I agree with Lilfishie, a 5.5G is only really big enough for a Betta on it's own. Some people have kept a couple of shrimps or even African Dwarf Frogs with their Betta but I'm not sure whether their tanks were of a similar size.