Is it best to just feed once a day?
Either once or twice, but if you feed twice use half the amount of food each time so the total amount per day is the same. I do realise it's next to impossible, but try to feed the same amount of food per day that is the same amount as one eye per fish. A fish's eye is about the same size as its stomach.
Feed a variety of food. The main staple should be flake or pellets (that is, commercially prepared dry food). They are designed to have all the nutrients a fish needs. If they keep spitting pellets out and trying again, soak them in a bit of water before putting in the tank.
Live food, or frozen live food, can be fed as a once or twice a week treat instead of flake. Frozen is safer as it kills any bugs you can get in alive live food. Bloodworm, brine shrimp, daphnia should be available as sheets of cubes in a freezer in most shops. The cubes go a long way. For the fish you have at the moment, cut a cube into four and defrost a quarter in some tank water before feeding. Dried live food is available but this should definitely be soaked before feeding to rehydrate. Dried has less nutrients that frozen or alive because the drying process destroys a lot.
And peas once a week. Microwave a few in a bit of water or save some from your meal (unless you cook them with a lot of salt). Squeeze the insides out of the skin and chop up or mash till the pieces are the right size for the fish's mouths. I feed peas the day before I do a water change so I can get out any bits left uneaten before they decompose. Even carnivores go mad for peas