If your water has low pH and KH, it should be fine to use. If your wild bettas have been aquarium-bred (which I recommend as they are friendlier and will more readily take pelleted foods), they are less sensitive than wild-caught bettas.
Since the tannins stain my water to a dark tea colour, I only use java moss as there is not enough light penetration to grow anything else.
The tanks that don't have any substrate are only my Betta splendens. All my wild tanks have substrate except for my fry grow-out tanks.
I have used Oak and Alder leaves in my wild betta tanks and had no problems with either. However, I use IAL as they look more authentic and tend to produce the highest level of tannins. '
Hope that answers everything for you.