So, to be clear, do you have the 6 gallon tank already? If not, you'll want to get it soon. If you really have your heart set on betta and neon tetras, it's wise anticipate a potential fin nipping situation. In that case, it's better to have a cycled tank on hand than a sterile one. Also, have you considered a female betta? They can be very beautiful (and less aggressive) without the temptation of the male's longer fins.
If your heart isn't set on a betta in your community, you could also select another species and not get the 6 gallon at all (or set it up as a separate betta tank). Avoid anything big enough to eat the neons (which would be inadvisable in a 21 gallon tank anyway). Some small species of gourami, small schooling fish like other tetras and rasboras, and invertebrates like snails and shrimp are all possibilities.
