EPS (expanded polystyrene) and XPS (extruded polystyrene) are literally polystyrene foam; Styrofoam is a registered trademark of Dupont (formerly Dow Chemical) for their closed-cell XPS.There are multiple kinds of foam insulation used for buildings. EPS and XPS probably closer qualify as "styrofoam." I believe they're as flammable as the regular packaging styrofoam. Polyiso is non-styrene based kind, supposed to have better flame retarding properties.
The polystyrene foam used for packaging is EPS, and not Styrofoam... but yes, any polystyrene foam is flammable.
"Polyiso" would presumably be polyisocyanurate, which is flammable but doesn't spread flame as fast as polystyrene foam; however, it produces lots of toxic smoke.