One week fishless cycle isn't usually enough to get the tank fully 'cycled', and the fish-in cycle isn't a great option, but if you are an experienced fishkeeper and willing to do the daily water changes, it *could* be okay.
As I mentioned before, I don't know lots about cichlids, but a 20g sounds small for the green terror unless you are willing to trade that fish in when it starts to outgrow the tank.
Also, most people on this site will tell you that your LFS staff know nothing. I quite like my independent LFS, but then again, I've received far better insight from the experts on this forum than from my LFS.