Tex,
As long as you use the flat latex straight from the can without adding water it will cover the Styrofoam fine. Usually, I dip the brush in water occasionally when painting the plaster, but not for the Styrofoam.
If you have any trouble at all with different materials (such as bare plaster, Styrofoam, Sculptamold, etc.) in the way they absorb paint, just give everything a quick primer coat of flat latex first.
Sometimes, I'll do the primer coat in a light tan or light gray, and then use dilute washes of acrylics for the final color.
But, if you want everything to match with no variation, just give it all a good heavy coat of the latex in the color you want.
Plaster, Sculptamold, pink Styrofoam, and Hydrocal castings in this scene:
With a primer coat of light tan flat latex:
Acrylic washes in some areas:
Finished scene:
Jim