It's recommended to use 1-2 lbs of live rock per gallon. You can use any of many types and could even use base rock (not live) and add 2-3 pieces of live rock to seed it.
The live rock is important to your system as it is your main form of filtration.
If you were to get massive show pieces of rock, the island idea could probably work while still keeping you within the 1-2 pound rule. But chunks of LR like that are very expensive.
You can go with all live rock or a mixture of dry rocks and live.The percentage of dry to live is of your choosing.I prefer at least a mixture of 20% live.
You can buy live rock locally and break up pieces to fit your aquascaping ideas.Of course,you're probably aware that there are alot of online vendors for live rock.Live Aquaria and Salty Critter to name a few.Calling Salty Critter to order rocks,they and some other companies will work with you on getting the size rocks you are looking for.