I couldn't find what the bottom of ha long bay looks like, but I will try to answer your questions. I would add that in a tank that size, I would plan on 2 canister filters.
1. All mosses are low tech. It's a matter of what you are looking for. Moss can be messy and end up in places you never intended. I have weeping moss in my tank that I never even put in there...haha. With a tank that size, you could use dwarf baby tears, or maybe something like anubias nana petite. I do not know what kind of grass you are referring to, but most grasses only do well in high tech setups.
2. I would need a better idea of what you are looking for. A plant list would be helpful.
3. CO2 is used on high tech setups. It is still undetermined if it's even necessary. There is also know way of knowing consumption of CO2.
4. Any plant on a rock will spread to the substrate. You will need to do regular trimmings to keep your plants where you and them.
I hope I helped at least a little. Best of luck.