Sometimes they have lotsa them so you have to go often to check out what stock they have. Depending on the size, I have 1 yellow tail dogface for about $3.50 and the other spotted dogface for about $7 - both also about 2inches long.
I have 3 of them and they are together for about 5 months and they still peck at each other once in a while... seems to be normal - at least its not fatal. They smallest dun seem to grow much but the dominant one is quite big from the time I bought it.
I'm no sifu or senior... IMHO, there is actually no rule to say that 1 feet depth is no good. It depends on what you want but it'll definitely look better if you are planning to have FOWLR. If you plan to have only fishes then it's really up to u.