In my experiences with mosquitofish, they do fine alone.
A while back, we purchased 5 mosquitofish and 2 minnows from our nearest Petco.
We stored them in a tiny fish tank to use as feeders for our turtle.
The minnows died some time afterwards. (I wonder if the mosquitofish killed them?)
So we went back to Petco and purchased more minnows, most of which were much larger.
We upgraded the mosquitofish and minnows to a larger tank, since they were in an undersized tank before.
At this point, we were just keeping the fish for the fun of it.
But apparently, the minnows were carrying a disease, which wiped out all the fish except for one female mosquitofish.
As of now, we are keeping that female mosquitofish alone, as sort of a pet or something.
She's been seemingly content alone, although she might be more bold around a few others.
However, female mosquitofish can store sperm, so there's a risk that a single female may be pregnant from an interaction with a male.
I personally believe that a combination of mosquitofish and fathead minnows would work best for mosquito control.
Fathead minnows forage for larvae among the bottom detritus, and mosquitofish devour any that come for a breath at the surface.
But since mosquitofish are so aggressive, I'm studying the conditions required for the two species to coexist.
Supposedly, koi and goldfish are better at controlling mosquitoes than mosquitofish, but I doubt that.
Mosquitofish are extremely attentive predators of moving, wriggling prey. Carp are more like indiscriminate bottom cleaners.
