Is it possible to control the polygon count of Unity's graphcis primitives?

Title pretty much says it all.

Not really, no - you're generally best making your primitives in a 3d modeling program

If you just want a low polygon plane creator, there is one on the wiki:

http://www.unifycommunity.com/wiki/index.php?title=CreatePlane

The primitives are models made in a 3D app (Maya, if I recall correctly). They're not mathematically generated in Unity or anything.