I do not know how exp is distributed.. I got different exp even if the same action and result is done by 1 pet..
I know on Pete's cove (2nd stage on the very 1st island) is 400 for non-participating pets on the battle..
But if I get perfect attacks with 1 pet, I would get 550..
Then with the same pet, I get 415, 435, 475...
I assume the exp score do not pick up every fight?
I have been leveling all the pets (got to 36 and slowly up to 37) .. I have done this numerous times to keep things within a controlled environment... I thought the stars will determine exp but did not (tried and I got more exp on some battles than perfect battles with the rough amount of damage)
I find the scoring very annoying.. To get a total of 1000 damages = 416 - 678 exp for one super rare pet (with 3 attacks)