Moist sandy gravel between frost-proof monolithic slab and insulation. Is it going to freeze and expand?
Quoting from "Common Questions and Answers about Frost-Protected Shallow Foundations" on page 29 of the Federal HUD guide on constructing frost proof shallow foundations dated June, 1994:


  Question No. 1:  How does insulation stop frost heave from occurring?
  
  Frost heave can only occur when all of the following three conditions
  are preset:  
  
  1) the soil is frost susceptible (large silt fraction),
  
  2) sufficient moisture is available (soil is above approximately 80
  percent saturation), and 
  
  3) sub-freezing temperatures are penetrating
  the soil.  Removing one of these factors will negate the possibility
  of fromst damage.  
  
  Insulation as required in this design guide will
  prevent underlying soil from freezing (an inch of polystyrene
  insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on
  average).  The use of insulation is particularly effective on a
  building foundation for several reasons.  First, heat loss is
  minimized while storing and directing heat into the foundation soil --
  not out through the vertical face of the foundation wall.  Second,
  horizontal insulation projecting outward will shed moisture away from
  the foundation further minimizing the risk of frost damage.  Finally,
  because of the insulation, the frost line will rise as it approaches
  the foundation.  
  
  Since frost heave forces act perpendicular to the
  frost line, heave forces, if present, will act in a horizontal
  direction and not upwards.


So you are OK as long as your non-susceptible fill does not contain much silt.