You could try a sand sifter goby. I have a diamond shrimp goby and he's adorable. They say they can be tough to feed if you have more agressive feeders in the tank, but mine is doing great. Even if you have a load of pods (which he will love) you may not always have them, so be sure to get him eating some mysis too.

You'd def need a fitted lid -- mine is named 'Houdini' for his escape attempts after dark. Another plus - he will keep your substrate aerated.
I also have a 6 line wrasse in the tank, and hes not a very aggressive fish overall [I have a peaceful tank] but he is fast and will outrace slower fish at dinner time. Beautiful and interesting fish, but yeah, I could see how there might be an occasional agressive one.
I've been getting my fish at Beldt's because other places I had been going before that were um, well, lets say the fish were just not healthy at all. Every single fish Ive purchased there in the past several months is a healthy fat chowhound, haha. Anyway, I recommend them for a 6 line. They usually have a few in stock, and diamond gobies as well if you want to have a look at one. I think a 6-line is about $20-25, and about the same for the diamond goby.
Hope that helps!