Flakes aren't a very good food for bettas. They're generally poor quality and they're full of air, which makes them difficult to digest. I suggest switching to a high quality pellet food such as OmegaOne Betta Buffet Pellets, Atison's Betta Pellets, Ken's Betta Crumbles, and New Life Spectrum. Having the food in pellet form makes it easy to avoid overfeeding. Fish weren't designed to eat dry, air-filled foods, so soaking the pellets in tank water before giving them to your betta is an easy way to prevent bloating and digestive problems.
I suggest shopping online at http://www.fosterandsmithaquatics.com
they offer flat rate shipping and their prices are at least $10-$20 lower than you would find in pet stores.
As far as tank mates go, if you get any kind of schooling fish you should make sure you get a minimum of six individuals. Pygmy cories are great candidates because they are adorable, they shoal together and give the tank a lot of movement--their small size allows you to get six individuals without putting too much strain on your bioload. This would probably be one of the best options.
Frogs are great, too, but they can be exceedingly difficult to feed and keep happy. They like to be in groups, and are nearly blind, so they hunt by scent. The only way I could feed mine was by dangling a live blackworm in front of its face until it realized it was there. Very annoying.
Platies and mollies are not good tankmates, in my opinion. They prefer much harder water with a higher salinity than bettas would thrive in. They're also aggressive and could harm a slow-moving betta. Male guppies are so colorful that the betta may mistake them for other male bettas--dull females should be ok though.
Remember to research any fish you're considering getting for your aquarium. They all have their own unique needs, don't expect a pet store employee to explain them to you.
Also, I suggest reading about the nitrogen cycle, if you have not already, and how to conduct a fishless cycle.