i'll give you some advice you probably didn't expect, but this comes from years of experience. if it ain't broke, DON'T FIX IT. cuz if you try, and something goes wrong, you WILL be blamed.
if that powder blue is doing fine with his feeding regimen then i would NOT upset the balance. i would just use the sheets of noori and put in clips for spinach and romaine, and this would be the ONLY change i would make for the time being. this will create the need for lots of water changes, but that just means more $$ for you. however, powder blues are NOWHERE near as dirty as nasos, be glad for that! those fish are practically sh!t machines.
if the owners don't want to worry about the tank, you set up your own parameters/rules with a contract for doing them the favor. put everything in writing - get them to write down their expectations, you write down what you're willing to do and be absolutely certain that it's all agreed upon before you sign. i always have one hard and fast rule, and if the owners break it then that's it, i'm outta there: THOU SHALT NOT ADD ANY SPECIMENS OF ANY KIND TO THY SYSTEM WITHOUT MY PERMISSION. this is the one thing that has caused the me the biggest headaches every goddamned time, almost without fail.
also, be sure that they have coverage on their homeowner's insurance, and you want to be certain that you've covered your own butt. this means that you must not make big changes all of a sudden in the sys.
i agree, the bird wrasse won't do well in the long run in that tank, unless it's set up for more circular swimming. but that could literally be a year or more in coming. deal with it when it actually becomes a problem. you'll just need to wean them off their fish, bit by bit, and then replace with specimens that won't require so much swimming room. for example, Coris gaimard (the coris wrasse) is a really hardy and always colorful substitute. but, truthfully, if the tank is well established, save yourself the headaches of making changes right away.
then, don't forget to charge up the butt. (j/k..slightly)
the wrasse, i thought, was called a bird wrasse. yeah, they need room and they do get big. the powder blue has been going along for 6mths you said? so, it's gonna be fine, imho. your problem will be in the long run if it does too well. then it'll get big. believe it or not, that 3x3x3 ends up being about a 200gal tank. not the best dimensions in the world, but having worked on one almost exactly like it some years ago i can tell you that it's not so bad.
so, here's my hard-learned advice: first, YOU decide what you're willing (or not) to do/put up with. maintenance can be a real easy source of hard cash, as long as you don't make things hard on yourself. this fishkeeping thing is as much an art as a science, so when folks tell you something absolutely can't be done, and you see that it is being done, learn from it. you can make this as easy or difficult for yourself as you want, i vote for easy. (thus, the "if it ain't broke" advice.) if you want more information on how to do this quickly, easily, safely, and for profit just p.m. me, i'll be happy to share what i know.
, and, you guessed it, a powder blue tang 
