I would say you are probably at the limit of your big fish! Four tangs in a 150gallon isn't too many from what I've seen of local tanks doing well, but the sailfin is going to get to be a very big tang.
As for small fish... I'm sure you could add a few more. When you consider the bioload change of adding a clown-goby compared to the tang... Well... The tang emits excrement the size of the goby every day :P You just want to make sure anything you get doesn't aggravate the bigger fish! If you added more chromis for example they like to swim a lot, and that will take up water column. But if you add clown gobies, those little guys never leave the rocks except to lunge for food and back. At least mine don't.
As for tank cleanup, you have a wrasse and you have a diamond watchman goby. You could maybe try for a sand sifter, like a banded goby of some kind or other, if you find your sand isn't getting stirred up. Inverts would probably accomplish it better, but I don't know how compatible those are with the rest of your fish!
Hopefully someone with a big tank can weigh in for.
Cheers, good luck!
