It depends on the size of your tank. Corydoras need to be kept in groups and need at least 30 gallons so they can school. Neons also need a 30 or larger tank and would probably be ok with a betta, but they might be attacked in a smaller tank, and anyway that's too stressful for them. The size of a fish isn't necessarilly an indication of the size tank it needs for optimum health.