Can you use Poster Paint for fantasy miniatures?
I strongly advise against using poster paints for miniature painting.

Poster paints are described as (emphasis mine):


  a distemper paint that usually uses a type of gum-water or glue size as its binder. It either comes in large bottles or jars or in a powdered form. It is normally a "cheap" paint used in school art classes. 


With the correct primer the paints should be able to stick to the miniatures, but in contrast to acryllic paints they stay water soluble after drying. They are vulnerable to moisture and abrasion and rub off while handling the miniatures. A protective sealing coat could help, but any sealing (either spay or brush-on) is a liquid, which starts dissolving the paints again. You'd need to very carefully spray several thin coats of sealing or you risk your paints bleeding and loosing details.

The biggest problem, though, is the quality of the paints, more precisely the pigment density. Poster paints are usually cheep and intended for kids to paint on paper. You can have a whole lot of fillers and just a tiny amount of (expensive) pigments in those paints, because all they have to do is cover a white piece of paper with a colored paste.

In miniature painting, you want to apply coats of paint as thinly as possible to preserve all the details in the miniature. You may want to layer different colors or highlight a detail on a dark background. You need paints with a high pigment density for this job, or you'll be forced to literally cover the miniature with a colored paste, just to paint a white dot on a black background. That's usually the reason why dedicated paints for miniatures are as expensive as they are. They contain much more pigments than your average hobby paints.