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 require other members of their kind. The lack thereof produces stress, resulting in a life-span which is shorter than it needs to be. (See 15 Stresses In the Aquarium).
All of the tetra, danio, and rasboro species are schooling fish in nature. That is, they live in large homogeneous 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 of an aquarium with a single female of the same species, will probably be much too aggressive towards her. They should be kept in trios consisting of 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 affections.
Barbs, dependent upon their tankmates, may be guilty of chasing and fin nipping. However, in groups of five, six, or more, they are generally content to confine this “aggressive” behavior to themselves as a form of play.
Loaches and cory catfishes, while not being schooling fish per se, are very much of a 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 fair or healthy to many of the species you have chosen.
As an entertaining and interesting change, you might consider a species tank. That is an aquarium stocked entirely of one species, with the exception of cory catfish or loaches as scavengers, and perhaps a plecostomus to perform some of the janitorial duties.