a 50% water change is to large of a change. i personally only do about 5 or so gallons on my tank and we'll say its about a 100 gallon total water volum which is only a 5-10% change. you would be better off doing a few smaller water changes more frequent then to do less often larger changes.
infact, if you properly setup your tank, you shouldnt have to perform a water change to remove nitrates but to rather only replenish trace elements that have been consumed in your tank. this is benificial to both corals and fish.
as for macro algaes, is there a reason you wish to grow your own? if your looking for a cheap way to feed the tangs, asian food stores sell like 50 sheet seaweed packs cheap ( the kind they use for sushi ) which your tangs shouldnt have a problem eating. if your going to grow a macro algae, i suggest just doing chaeto in your refugium for nutrient absorbing.
as for the moorish idol, look into long fin banner fish as they have a similiar look.
as for the red sea max as a QT, ofcourse it will work but it wont be much of a "display" for another room while being use as a QT. the tank is best bare bottom, with a few peices of PVC pipe for hiding so it wont be very pretty to look at. its also best to put it in a low traffic area as to not stress out the fish.