Very nice rock arrangement. I'm curious, how will you keep the PH down? With that type of substrate and that many rocks I imagine it'll go up to 8 without any co2 injection. If that's the case, African Cichlids might do well =)
I doubt I will have particuarly hard water. I have used this substrate and these rocks before and it didn't effect my PH. I have not decided on inhabitants yet but I doubt I will have to worry about anything, except for anything that requires very soft water.