Honestly, I don't see anything morally wrong with it. I'm pretty sure they're are plenty of mantises out in the wild eating vertebrates. Yes, their main diet are invertebrates, but I'm pretty sure if they had the chance and a vertebrate walked right in front of their raptors, they would snag it. I don't know, that's just the way I see it.
Anyways, I'm not going to debate whether it is morally right or wrong; everyone has their own set of beliefs. My main focus when I created this thread was for the sake of my mantises's health. I thought may be the bones could mess up their bodies, or fish don't have the proper nutrients, or can mantises get food poisoning the same way humans get salmonella from eating raw/undercooked meats. I wasn't planning on making fish a major part of their diet. Just something to snack on here and there since the minnows in the pond near my house can be easily acquired at no cost.