ok i seriously doubt you will have any cold issues bare in mind your fridge is very well insulated, IT WILL KEEP THE HEAT IN the prob you will have is getting the heat out remember the bulbs stay hot for a fair while after their off. i prefer my favorite soil is canna cocoa i use this for indoor with Canna A and canna B feed (you have you use both). this soil has achieved best results for me. i think your best bet would be to perminantly fix your light to the fridge top and use the shelves that where in there before to change height.