my aggressive ex-community betta was placed in a 7 gallon, i decided to add a few pygmy corys (3) and hes fine with them, i removed the betta into a jug, did a water change, floated the bag in the water,etc,etc added the corys then the betta, by the time the betta had been returned the corys had made them selves at home, the betta didn't bat an eyelid.
your stcoking options i personally thin in a 5 gallon along with your betta could be 2-3 of either corydoras pygmaeus or corydoras habrosus. and maybe a couple of shrimp, shrimps dont add any waste to the tank that you will notice and their for only room to move and tank mates is an issue (aposed to adding more strain on the filter)or a couple of ADF would go nicey instead of the shrimps, they require a little work and im not sure about them being housed with corys but what ever you want, its your tank,