I dun think that weighing individual resists is feasible. You would have to weigh those for the individual pet, not across the board.

For example, a CU can have max 45 fire, a Hiryu max 90. If you weighed fire resists as more important, then the CU would always have a lower value.

One would be tempted to give more weight to the intensity of the lower resists of a pet than the higher resists, for more even balancing of the pet.