TwilightNite, yeah, even without a filter you should be fine with 30% twice a week. You only have 2 bettas in there so your bioload is still pretty small. And once your betta's fins heal up and there's no chance of infection, you can start doing 30% changes only once a week. But until his fins heal, definitely aim for twice a week.
I'm not sure about the plant. You'd have to tell me what kind it is but I'm not a huge plant expert, especially terrestrial plants. You could post on PewPewPew's plant thread, she might know.