Shoaling fish live in large groups; this gives them "security" (safety in numbers idea), plus depending upon species they may have quite a social structure, or a "pecking order" within the group and these are important for the fish's health.
This is a very different thing from "schooling" which is what many assume to be the issue, but in fact it isn't. No freshwater fish (that I am aware of) "school" in the true sense of the term, as marine reef fish do. Swimming together is just that, swimming together in the aquarium, and shoaling fish may
sometimes swim in a group--some species do this a lot, such as rummy nose tetra, cardinal tetra a bit less, most rasbora, corys, loaches, and others. Loaches are a fish group that have a highly-defined social structure and the fish must have others of its species.
The effect of not having a group varies from species to species, and sometimes even fish to fish. Sometimes when I have had a group of some characin species for many years and they begin to die off of old age, if I do not particularly want more of them I just let nature takes its course. The last fish remaining seem to be fine, and if they have reached their normal life expectancy presumably they are. But starting out with a shoaling fish, I would always buy them in groups suitable for the tank space and the species needs.
Sometimes adding new younger fish to a couple of old timers of that species can be a worse problem.
Depends upon the species, and the fish themselves.
If you can find them, and like this fish and want a group, and have the space, I would. If the answer is no to any of these factors, then I would not risk upsetting the two. Other characins should work fine with them. Just bear in mind one point though, that this species can sometimes be a bit nippy (the fins of other fish). It is a trait of species in the Aphyocharax genus, as noted in our profile.