Most saltwater species cannot be kept in an aquarium with their own kind. In the freshwater world, things are totally different. Many of our more common home aquarium species actually need other members of their own kind to feel secure and safe, otherwise they suffer stress and a likely a shortened life-span.
All of the tetra, rasbora and danio species are schooling fish in nature. That is, they live in large, homogenous groups with a designated leader and individual purpose. They should not be kept in aquariums in groups of less than six.
A single male livebearer kept in the confines with a single female will probably be much too aggressive towards her. They should be kept in trios with one male and two females or in haphazardly produced groups where the females largely outnumber the males. This will allow the male to divide his attention.
Barbs, dependent upon their tankmates, may be guilty of chasing and fin-nipping. However in groups of five or more, they are generally content to confine this “aggressive” behavior to themselves as a form of play.
Loaches and cory catfish, while not being schooling fish per se, are very much social fish, and do not fare well individually. Both should be kept in groups of at least three to insure their long-term well-being.
Often, with the diversity of colors and body-shapes available, it is tempting to stock your aquarium with one of everything. This is not healthy or fair to many of the species available.
As an entertaining and interesting alternative, you might want to consider a species tank. This is an aquarium that is stocked entirely with one species of fish, with the exception of cory catfish or loaches as scavengers and perhaps a plecostomus to perform some of the janitorial duties.