LOL.
yep some places put the size tank at what I would call small when you look at the adult size of the fish. Unless they are thinking you'll never kep him alive long enough to reach adult size.
There are a lot of different theroies on Ich and feeding habbits. I have my own and won't go into them here.
I will say this, I have a pair of Naso tangs, male and female. They are in a 7' long 400 g tank. To watch them swim around, it sometimes seems that even this tank is too small for them. The male is about 11" plus his streamers.
As for Ich, STRESS is a major contributor. In the last 7 yeasr of having my small reef, I've only had two cases of Ich. the last one recently. I don't QT but I do pick my fish carefuly. The clown was a gift from a friend. He had Ich when I got him. It was cleared up in less than aweek in my tank and I did nothing. No other fish got it.
Live aquaria and I beleive marine Depot tell a bit about the husbandry of the as wel as sutiable tank size. There are some tansg that don't get all that large. Try surfing their websites and se what you come up with.
As for the triggers, I have a pink tail in my 400 G tank. He's beautiful and has a great personaility. He dosne't bother any of the snails I have in the tank and can't get into the rocks for the shrimp. Not that I think he would eat them. He has been in capitivity for about 3 years now, along with the tangs, and has just recently be exposed to snails.
The previous owner had the tank as FOWLR with no inverts and so the fish have never seen them before. Or at least not for a long time. I have noticed that they have been conditioned to eat sinking pellets as thats all they've known, besides the occasional sheet of nori, for the last several years. I've put frozen food in the tank that will float and they totally ignore it. If I thaw it and then pour it in where it will sink, they devour it. Just some of my observations.