1.) Aquarium salt is good to keep in tanks, even if they're not sick. It helps to ward off infections, such as fin rot, and helps to produce the beneficial bacteria "slime-coat" on the fish. I swear by it. Just be sure your pH isn't too high, and you'll be alright.
2.) For a 5 gallon with a filter, I'd do a 15-25% water change every week. Take a gravel siphon and suck out uneaten food and waste from the tank, and replace it with clean, dechlorinated water.
3.) The buffet bites and freezedried bloodworms are a good idea for the betta. You can try to feed him flakes every once and a while, but it isn't much of a treat. Some bettas will stare at the flakes, then stare at you, like "Seriously? You expect me to eat that?
You can also thaw out a pea, shell it, cut it into small bits and feed it to him. Although bettas are carnivores, they need their greens like we do! It will help keep his system in check and is pretty inexpensive.
Hope I helped.