I agree with Avi. You can certainly add the liverock now. There's really no need to wait. Actually, if you do wait, you may re-cycle if the rock isn't fully cured or cured at all. Adding it now would probably be best.
As for the goby, I have never had one. But....and I don't know how true this is, but I have been told that for most "sand sifting" fish, you need a mature tank of at least 1 year otherwise, they will pick your sandbed clean. Again, I don't know this to be fact, as I have no experience. I'm just passing along what I have read on other posts.