Kudos to you for getting a proper home for your bettas, they will be much happier. Anyway, I don't know exactly what that kit includes but you'll of course need a filter. Also, bettas are tropical fish so you'll need a heater suitable for the tank. Regular water changes are vital (15-25% weekly) to keep ammonia down. You really need to let the two tanks cycle for a few weeks. This means letting them run without fish for as long as it takes (usually 4-6 weeks) in order to develop beneficial bacteria. Bettas will probably be fine without a cycle though honestly, just keep up with water changes. After the tank is established you could maybe try to add a few small tetras. Neons are best. 5 would fit just fine in there, but no promises that the betta won't attack them. Another possible tank mate is an African dwarf frog or two. These guys are very entertaining and are fully aquatic. Finally, shrimp and snails, as with all inverts, are at risk of being eaten or attacked. Just provide plenty of hiding places to maximize survival chances. If you get shrimp or snails, make sure you get algae wafers or some other type of sinking food so they'll have something to eat besides the betta's waste...haha. It's worth noting that you should probably add anything you might want to the tank before you add the betta, so the betta won't establish territories with himself in mind. Adding him after everything else would reduce the risk of him attacking.