Big update today! I now have nice 6x6 back sprites working for all Pokemon! No more gross, pixelated back sprites! Along with the other graphics and palette changes to the rest of the battle HUD, my gen 1 battle screen looks almost identical to the batte screen for Gold!! :D
All the credit for the working 6x6 back sprites goes to Sawakita for showing me how to implement this! I never could have done this on my own!