cowfish cautions

[Aquattro]

It sounds like there is plenty of swimming room for the fish. Normally you wouldn't put that many tangs in there, but I have a feeling they are smaller than they could be and if they're getting along, great. The trigger can get really big and eat things you'd rather it didn't. The clowns and damsels are going to harrass anything you add, just because they're like that. The wrasse sounds like a lunar, which can also get fairly big (and hungry).
One of the concerns with adding more livestock is whether the filtration can handle the increased bio-load. Can you post pics of the tower and skimmer?
If it were me, I wouldn't add more fish. Certainly not any tangs! Ultimately you need to figure out how good your filtration is and can it handle more. If the tangs now have been there for a long time and not properly fed (nori, etc) they may be stunted and not take up as much room as they should. They can get to 8 inches or so. You can also figure out any areas that don't have fish and maybe find a fish that fits that niche. Let's see what others say!!