If you're set on a betta, that's cool, and a pretty good choice.
Some other choices for upper middle fish would be-
1 dwarf puffer (they're freshwater, tons of personality. Limited on plant choices though...Plus they're territorial, but will probably leave things on the substrate alone. Unless it's an invertebrate, then its lunch.)
1 Badis Badis (beautiful fish, and can normally only be kept in a species tank- a 5g would be perfect.)
a school of 6 ember tetras- (they're tiny, maybe 1/4-1/2 inch long- and bright yellow/orange so you can't miss them in the tank.)
2 of the smaller gourami species. (sparkling gourami and croaking gourami are smallish- 1-2 inches long.)
2-3 male guppies/mollies/platies (livebearers also graze on algae, so you could, theoretically, use them as your cleanup crew.. swordtails like more room I believe)
Options for lower strata/cleaners-
3 pygmy cories
Snails (assassin snails or nerite snails would work well, but not with the puffer. If you choose the puffer then pond and ramshorn would be best, since they will multiply, are usually free, and will be an alternate food source when the puffer feels like hunting.)
Shrimp (RCS and/or Ghost shrimp would work well- personally I love shrimp as the cleaner crew over corys and ottos. The only thing I feel is better than shrimp and snails at cleaning IMO is the BN pleco, which won't fit in a 5G. Works ok in a 10G though if you decide to move up. :))
Really, the decision should be based on the PH/hardness of your tap water (rather, the ph/hardness of the tank once cycling is over. Snails/livebearers/or I believe the puffer would prefer PH above 7.
the tetras/gourami/badis badis/cory like acidic water, around 6.
If your water is around 6.8-7.2, then just about anything will do pretty well.