[Bombalurina]

A 50% water change would be fine. With live plants or an oversized filter, 25% a week would be fine. Just leave your betta in, same as you do your neons.

My female bettas are in a community tank and they hold their own pretty well - actually, they feed first and everyone else just gets what they cause to fall to the bottom in their feeding frenzy. I feed a combination of micopellets and betta micropellets (I have micro-fish species in with my girls) and everyone gets a bit of everything. Specific betta pellets are generally better than flakes, though, as the meat content (bettas are pure carnivores) is much higher. Generic tropical flakes are generally aimed at omnivores and have a high vegetable content.

 

[Bombalurina]

Dried bloodworms will be loved by both bettas and neons, but they aren't good as a staple diet. It won't matter if you just feed pellets and flakes - both species will eat both. Flakes won't harm the betta and pellets won't harm the neons, but it also ensures that they are getting the nutrients they need through the food appropriate for their species.