I'm afraid not. You're already completely on your limits in that tank. Also, a red tailed black shark gets up to 5 inches with ease, they require a tank much bigger than yours. I'd say a 55G at least. Stick with the fish you have for now, and then in the future get a bigger tank, and only then start thinking about getting more fish.
--edit--
About the water, you will be ok leaving it overnight and then putting the water in tomorrow, but it's not the best solution. That's why we've been telling you to buy water conditioner as soon as possible ... this will get rid of all the other toxins in the water as well, which just letting it stand overnight won't do.