the algae blenny will NOT compete with the mandarin for food, scooters maybe, but not algae, they get their name from the fact that they eat algae, they are strict herbivores, and mandarins primarilly eat pods, so that is not aprob, as for the blenny, some say he may be aggressive if added first, but IMEO they have been no prob for anything except my tang, and only because of food, if one eants what the other wants, they push each other a bit(both equally) and it is kinda cute, but no damage to either, and ours was the very first fish added
you said it, a 45 hex(especially) is a bit small for a tang, they really should have 4 ft or more of swimming area, it is not a matter of out grownig the tank, as much as swim space, so i would avooid gittingone