I actually have a 5.5 gallons tank too, and hosting a corydoras catfish, a pleco and of course, Clown, my beautiful betta.
I have a very powerful filter (penguin 170) and the Betta really like to fight the current, swimming against it.
He has spots where he can rest and where water doesn't move almost, but generally, I see him swimming around

.
Even if they are so easy to keep with minimal filtration, I believe that they are happy with space and good quality filtered water.
I also have a 75 gallons tank with cichlids (convicts and managuense) and a medium size pleco. Fishes are great !!!