That's a cool pic, I love the tree. Looking forward to see this one filled.
With your substrate, I'd say you can skip the additional substrate ferts for a while. Doesn't look like you are planning to have heavy root feeders anyway.
Regarding other plants, that's a tough one. Nothing that grows too tall to hide or take away from the wood. Perhaps some Mini Pellia attached to the branches to give it a bit more of a "tree" impression?