just get a pond liner and build a 2x4 frame with some plywood. easy and whatever size you want it to be. yes the insulation on the pond is necessary. unless your garage stays within a couple degrees variation all year long.
Insulate everything. The wood frame w/liner will be much eaiser to work with. Opposed to a bulky round pool. As much as I love rubbermaids, they too are bulky and awkward at times.
but I'm thinking the 2x4 frame with a liner might be your cheapest bet. Then you can just put insulation inside that frame, drop the liner in, and get your pond on. you can really make the dimensions whatever you want.