I'm sure you've heard it enough about the Tangs, this isn't the first time I saw you post pro actively that you are planning a larger tank. I won't even go into it, as well, why make enemies

You have a pretty fair bio load, although the fish are still small. You still don't have much rock on hand to deal with that load, and that's why you want more. Problem is, adding rock that is any less than 100% cured will severely tax your existing biological filtration, most likely to the point of adversely effecting your animals.
Your best bet would be to find someone you can trust, preferably someone you know personally, or at least people here vouch for that's selling their established rock. Taking that route allows you to put that rock right into your tank without worries.
Buying live rock in a LFS, I would not trust ANY, I mean ANY LFS to sell me fully cured rock to put directly into the system you have right now. Put them on the spot, if they tell you it's 100% fully cured, and ready to put in your tank, ask them to let you borrow their test kits, and test the water. If their rock truly is ready to go in your tank, you will find ZERO Ammonia and ZERO Nitrite. Not almost Zero, I mean absolutely no reading. I don't believe that will ever happen in a LFS under normal circumstances.
I'm planning on breaking down my tank, and getting rid of my Fla. rock. I'll give you nice pieces, some with hairy mushrooms and polyps dirt cheap, but the hitch is, I have a flat worm problem. Unless you already have red flat worms, I don't think I want to pass this rock on to you, it wouldn't be right.
How much you need depends on how much bio load you eventually want in the tank, and which rock you want.
Aquacultured Fla. rock is nice, because they don't hurt the reef to get it. I bought twice from
Gulf View, and liked them a lot.
There is another guy doing the same thing, I think it's
www.tampabaysaltwater.com They get loads of good reviews as well. These guys sell rock that is heavy, and dense. Not as much surface area per lb, and because of how dense it is, you need more pounds to make a tank look like it's got enough rock in it. It does come with hitchhikers that you just don't get from rock that's been shipped from the other side of the planet. Some, people would prefer to avoid, but lots of good stuff with many colors. Many of those things don't survive curing, and most others don't seem to make it long term to live in our tanks though.
Rock from Figi or Marshall Island have a completely different look. It's much lighter, so it takes more to get the "pounds per gallon" thing worked out, but it's more porous, and you get better surface area, so you actually need less pounds per gallon when compared to the denser Fla. rock.
The best advice I can give you at this stage, is go out and look at tanks. Go to LFS, and check out <b><a href="http://reefkeeping.com" target="_blank">Reefkeeping Magazine</a></b> TOTM series to see what rock appeals to you. I'd tell you the same thing about lights. It has to look good to you.
Once you have a better idea of what you're looking for, helping you find it is easy
