120g or 150g is fine. As for tankmates, I had mine with Serpae Tetras before and found a few of them with one eye. I had some Buenos Aires Tetras in my planted tank to find all the new growth being nipped away. I think you would find Neons dwindling down as time rolls on. So it might be trial and error to find the right Tetra for you. I love Congo Tetras, but not sure how they would be with a BGK (should be fine, but not sure if they would end up like the Serpae). If Tetras aren't set in stone, Denison Barbs would look great with a BGK! Angel Fish and Discus also come to mind.
As for decor, lots of hiding spots so it can explore and pick it's territory. Plants are always a plus, but I'd focus on the hardscape making sure the rocks aren't jagged...smooth stones/boulders with some driftwood and simple plants such as Anubias and Java Fern always looks good. Any substrate will do. I prefer sand and I would go aquasoil (even though it's a bit pricey) over clay planting substrate just due to the soft/smooth texture.