Rock is mostly your preference (as long as it doesn't have anything in it that could hurt your tank) and there is nothing wrong with mixing. The general consensus is that porous rock is better...
Mine is mostly fiji (from various sources including marco rocks) with a little bit of TBS rock (aquacultured rock from the bahamas I believe), tonga branch, and marshall island mixed in.
Honestly though, my choices were more economical then anything.
From what I have heard, tonga is no longer allowing exports, so everyone better get their tonga while it isn't even more ridiculously expensive.