You are better off ordering off the internet instead of buying from petco. You can get it a lot cheaper that way, and often times you can find it on a place like Craigslist. I would say the most popular is Fiji Live rock, but there are many types. I actually have 100 lbs of Carribean Rock in my tank and I love the way it looks. Check around on Craigslist or see if you have a Reef club in your area. People who are looking to get out of the hobby, or downsizing will often sell it at a great price and once more it was taken from an established system so your cycling time will be greatly reduced. If not, check around the internet for places that sell it. You can find a lot of places that sell it and ship it to your front door much cheaper than you can buy in a store locally.
Are you using RO water to fill your tank? Tap water contains a lot of stuff in it that is not good for Saltwater. A lot of pwople put a RO system in their house for that purpose, but you can also buy it in bulk at your local supermarket, Walmart, etc. for 30 to 40 cents per gallon. Just bring your own containers and fill them up.
Do not worrry too much about your pH right now. Between the salt and the Substrate, it will eventually buffer out to the desired level.