Pin holes in leaves tend to mean some sort of nutrient deficiency. The cichlid in your picture looks like a mbuna. They can and will eat plants. I had mine with some anubias and they destroyed them. Snails will eat any part of the plant that is dying or decaying. If part of the plant is unhealthy due to a nutrient deficiency then the snails will eat the unhealthy part.
Yep. I was about to say the same. I would either blame the Mbuna or having dead spots in your plants already since those snails won't eat healthy/live plant. As far as assassins go, I don't think you can ever find them in pet stores. You would have to order them from someone selling them on an aquatic site like this. I know Msjinkzd has some. I just sold off my extras or I would offer.