I didn't put many of the new plants in my smaller betta tank. Most would just get too big.
Speaking of filters, I have come to the conclusion that it isn't rotting wood causing all this mulm in my tenner. At least, not entirely. The new wood isn't getting soft, but there is just as much mulm to siphon out every week. Either I have too many snails in here pooping, or the sponge filter just doesn't pick up stuff well, even with the increased flow. I am trying to be more diligent about gently stirring things up off the substrate level when I siphon out the tank, to keep it cleaner.