Your previous one looked pretty good, I liked how you did the waterfall.
I wouldn't worry too much about how it looks now, once stuff starts growing in (especially if you can get moss to grow on the walls/background), it will be hard to notice/pay much attention to flaws and what not.
I wouldn't worry much about pots for the bromeliads- you can just stick them wherever, if need be, but a little bundle of sphagnum around their roots. They generally don't do well with wet roots, and a lot are epiphytic.
I'd avoid the CO2, it just sounds like it would be a lot of trouble, and any sort of water feature is just going to take it out of solution. The emmersed plants won't need it, and it's a pretty small space for some of those plants, so it can get pretty crowded quick. And your water space is going to be even smaller.
Anyways, your previous one looked pretty good, so I imagine you'll do a pretty good job with this one.