In most cases, the fridge/freezer are the biggest consumers of energy aboard a boat and anything you can do to cut that down is money
in your pocket. Most fridge brochures reccommend an equivalent insulation thickness. If you don't have the physical space for the cheaper, lower R-value insulation, you may have to go to the higher R-value stuff to meet the insulation requirements.
I don't know of a reason you you couldn't mix and match insulation, but the more the better, the most seems to go on the bottom of the fridge compartment, a vapor barrier is highly recommended, and an air tight, insulated top is always a good idea.