Uncured live rock is a HUGE no-no. I made that mistake and could not get my nitrates down no matter what I did. It was a neat piece of rock, and I really didnt want to toss it, but it was causing so many problems, so I finally threw it away. After a week and a half and two 5-gal water changes, my nitrates dropped drastically, and another week or so after that, I added my first fish. Cured live rock, though a little more costly, is MUCH MUCH better!! You don't have to deal with a large amount of die off. Most cured LR will have coraline algae (good stuff) growing on it (not something I have seen with uncured rock sold locally.) You only have a small tank, buying cured live rock wont be too expensive.
For top-offs, you just add ro or distilled water to the tank, do not add saltwater, to raise the water level to the desired level. The reason you dont want to add saltwater is that only freshwater evaporates from the tank, the salt stays in the tank, raising your salinity. By adding the ro or distilled water, you bring that sality (and water level) back to normal. If you can afford it, buy a refractometer, so you can get an accurate reading of your salinity/specific gravity and can accurately gauge how much of what kind of water to add. I know mine was around $50, well worth it!
Only small fish can go into a tank that small, and I would only keep it to 1 or 2 fish max.
SaG covered the importance of quarantine tanks pretty well. If you just have (or plan to have) 1 or 2 fish, I don't think its all that necessary. I have 4 fish in my 24 gal, and to be honest, I didn't QT any of them. (shame on me, I know) But mine are all doing fine. Just be sure your water quality is excellent before adding fish, that will help keep the stress level down.
Good Luck!!!
