Welcome - and you'll love a reef tank, only downside is its addictive quality!
Dry rock takes a few weeks to become live, and since you need to cycle your tank anyway, then I think you are fine starting with mostly dry rock and adding a few live rocks. But I'd caution you from either using rock from your friends tank or seeding dry rock in your friends tank. I can't tell you the number of stories I've heard about people who have picked up pests that way and then spend an extraordinary amount of time and frustration battling problems. I think you are probably better off finding a good LFS (probably not petco as they tend to use lava rock and not ocean rock) and buying a few lbs of rock from them to get you started along with dry rock. You'll want to aim for 1-2lbs per gallon for your DT. You can put some of that in your sump, but you still want a good amount in your main tank as it will provide space for your fish to establish territory, hide, and sleep, as well as places to mount coral at various heights in your tank :)
Also, just to clarify, are you planning on using your chiclid tank? If so, have you ever used any medications in that tank? Medications, particularly copper-based medications can leach into the rocks and under the seals, and is deadly for a reef tank. I personally wouldn't reuse the rock in there for a salt water tank since all sorts of fresh water stuff could have gotten into the rock, and 6 months from now it could contribute to all sorts of water quality and algae problems! As long as you've never used copper in the tank, then it should be fine to reuse, just wash it out really well with vinegar and water!
In saltwater, the best thing you can do is research, research, research. Its not a cheap hobby - be prepared - but IME, the better quality equipment you get up front and the more you do to get yourself started on the right foot rather than taking short cuts, the easier time, fewer problems, and more fun you will have in the hobby! I would say the first 4 months were the most time consuming for me, but now things run very smoothly. Check out the stickies and articles sections of this site - the info is invaluable! People here are pretty welcoming and will happily point you in the right direction, celebrate successes, and grieve the inevitable losses (hopefully few). So ask away!
And to the OP: Paste, in addition to what others have said, I wouldn't recommend getting your own rock directly out of the ocean unless you plan on bleaching, drying, and recuring it. Its going to have all sorts of stuff on it - some good, but more than likely you'll also pick up some not so good pests...
