Just like DV said, mealies have a lot of chitin and can cause impaction. Most big chain retail stores and even some small LFS that house pacmans will feed them anything that would "seem" appropriate but I can assure you that neither fish or mealworms are a healthy staple diet.

Arguably, fish can be used as a staple if you breed them yourself to ensure that there aren't any nasty parasites piggybacking on it but why go through the headache of breeding fish to feed to your frog. The most common acceptable staples I see around for pacman frogs are crickets, dubia roaches, and nightcrawler worms.

I prefer nightcrawlers as they are really nutritious as is out of a cup. I buy mine from a local LFS for about $5USD per dozen every other couple days, feed a few worms on day of purchase and put the rest in my fridge with a carrot or a potato wedge. I'm too squeamish for roaches in my house and gut loading crickets is too much of a hassle for me.

Edit: Also the general rule of thumb is to offer food at minimum, every other day to ensure a healthy eating schedule. Eventually as pacmans grow they tend to eat less frequently reaching adult stage. My six month old female cranwelli will eat three big nightcrawlers in one sitting, she's a slow eater, and then proceed to bury herself for the next two days.