It's not cruel. What do you think happens to the majority of fry in any normal tank? I've got platies in with my angel, and have yet to see any hint of fry... I have no doubt that the angel occasionally gets a treat, and the added bonus of the thrill of hunting down the food himself. It's good for them. You are probably already providing fry for the angel, if s/he's in with mollies and a platy.