I have a 55 gallon tank, and for low-light plants you need 1-2 watts per gallon. So basically you need at least one 55 watt bulb. I bought a fixture at Petco that uses two 50 watt bulbs of 9000K each. So therefore I have almost 2 watts per gallon. I don't know if all my plants are considered low-light plants, but they are all doing ok. My dwarf red lily and huge amazon sword are doing great, as well as my red ludwigia (which grows slowly but steadily). My micro sword is also doing well (stays alive and green but doesn't seem to be growing at all in the 3 months that it's been in the tank), and I have some water wisteria, small amazon swords, and an anubias nana, all of which are doing well. I use organic miracle grow soil with a sand cap, so the fertilizers in the soil help a lot. The plants might yellow a little bit at first while they adjust to your tank, but they should recover quickly. If they start melting in a few weeks that means your lights aren't strong enough or don't have a high enough K number.
Your betta should be ok with those tetras. I'm pretty sure neons are the only ones that get nippy, but don't quote me on that.