Your tank needs to be an at least 10 gallon to have fish tankmates. So is it? And yes, Neons can be with Bettas. However, there maybe some nipping here and there.
Female Guppies are best with Bettas. The male's colors can make the Betta think it's another Betta. In other words, no guppies with lots of color. Get pale ones or ones with less color. They might get shredded or eaten. O_O
Another tankmate can be Cardinal Tetras, Pigmy or Dwarf Cory Catfish, Platies, Mystery Snails, and others. If your tank is 5 gallons and lower, Nerite Snail if you have a algae problem or even shrimp such as Red Cherry shrimp or Ghost shrimp. Hope you find the perfect tank mate!! :)
BTW, make sure to put the threads you make into the correct sections. This one should've went into the "Betta Compatibility" section.