  We consider integrated photon pair sources based on spontaneous four-wave
mixing and derive expressions for the pump powers at which various nonlinear
processes become relevant for a variety of source materials and structures.
These expressions serve as rules of thumb in identifying reasonable parameter
regimes for the design of such sources. We demonstrate that if pump powers are
kept low enough to suppress cross-phase modulation, multi-pair events as well
as many other nonlinear effects are often also constrained to negligible
levels.
