The 6-line will outcompete with tha mandarin for food... which might alreadybe a problem since the move sice your pod population most likely has to start over. So if you go with a wrasse, I would remove the mandarin... or, give the mandarin to someone with an established tank already.
Of course, you might have it adapted to brine shrimp as well.
A powder blue is an easy fish to stress out, and I wouldnt put one in less than a 6' tank. For a 4' tank, I would stick with smaller zebrasoma (yellow, red sea purple) or bristletooth (tomini, kole) tangs. Maybe, just maybe, a pacific blue, if you like that type of tang. It will eventually outgrow a 90g, but it can take a while to get there, unlike many other tangs that grow large.
As for the wrasses, my only suggestion if you go that route is to go with flasher/fairy wrasses over 6-line or something else. They can be kept in groups, and will tolerate other wrasses being added... where a 6-line will not. As long as you start with at least 2 wrasses, then adding more later wont be a problem. FWIW, I hate 6-lines, but love every fairy wrasse I have ever had. Much cooler fish, and they pick on flatworms just as well if not better than 6-lines, if thats at all a concern.
