Shotcrete is a method of placing concrete and the properties of shotcrete are equivalent to those of cast concrete. The type of work you are describing is done in many cases with the shotcrete process and is commonly called “ratproofing.” As you have engaged an engineer, we would suggest you follow his advice and he should determine the thickness required. Providing a row of dowels around the stem wall to tie the shotcrete to the wall should eliminate any significant separation between the shotcrete and the stem wall. Shrinkage of concrete between the walls may cause some minor hairline cracking, but nothing to allow ingress of rodents or insects.