I agree with letting the sand age a bit before getting any sand sifters.
Another note about Diamond, and sleeper gobies, they tend to have a high mortality rate. This would be a good fish to quarantine and be given medications for parasites. I have had numerous diamonds that have done just fine for a few weeks and eat like pigs but just keep getting skinnier and skinnier until dead, or they do just fine and then dead for "no reason".
If you really want one, then I'd recommend gabbing one that someone else in the hobby is selling.
My recommendation for a 1st fish of the tank (if you like them), would be a blenny. Very enjoyable fish to watch.