A variety is good for them, but sometimes you can only feed one or two different types, which is fine.
Pellets are a great staple in food for them. There isn't any mess as long as you put in one at a time and see that they are eaten. 4-6 pellets a day is a fair amount.
Flakes are good too, sometimes they are very high in protein but it's harder to control how much you feed them. These are notorious for clouding up the water so it is important not to over feed them flakes. They can also have the potential to bloat the fish if your little ones eat too quickly and the flakes haven't had time to absorb the water. But use very little at one time.
Frozen foods such as blood worms and brine shrimp are real healthy. They last a long time in the freezer, just cut up a small portion to feed them. You can take a small amount of tank water and use that to defrost them. You can use your clean fingers or a tooth pick or tweezers to pick them up. Roughly 3 blood worms per meal is a good amount.
Freeze dried is a hot topic among hobbyists. Some feed them regularly, others recommend as a treat. It's up to the owner what they feel is best. Use a tiny bit of tank water and let them soak for a minute or so prior to feeding. Then feed as if it were frozen. They are well known to cause bloat, so it is best to re-hydrate them as best you can and keep an eye on it. 2-3 is a good amount.
It's best to spread the meals into 2-3, if you can even 4 times a day to help prevent bloating. That goes for all types of food. Some bettas are able to handle more food then others. My Zoomie eats a little more a day then the others because he is constantly zooming around the tank. (Don't let him know, but I think he has ADHD) It is also good to fast them for a day a week to let their systems catch up. If you see they have a full belly hours after feeding, you can skip the next meal to help them out. Or feed them 1 pellet or 1 of a different type of food so they at least have something, but not too much.
Some are picky eaters and you may find that one of your guys just won't eat a particular type of food. If you insist they eat it, then you can soak it in garlic juice. Either from freshly crushed cloves, or from a jar of garlics unseasoned. It is like catnip for them. Mine don't like freeze dried, and one doesn't like flakes. I don't worry about those as much as I prefer pellets, frozen and homemade.
There are a variety of live foods that others feed, but I don't go that route so I didn't list it.
Hope this helps some, best wishes!