My routine is that I generally buy tangs as juvi's and put them in our 55 gallon. When they grow noticebly bigger I move them to the 90, and etc. We have two other larger tanks also. If I feel a fish is too large for our system, whether inherently, or via her personality or behavior, health, etc, then I rehome that fish to another aquarist who has a bigger system.
To make a blanket statement that no tangs, powder blue, whichever, can be kept in a 75 gallon tank is IMO not that way to look at it. I don't see a problem with putting a small tang in that setup, especially with a sump and fuge. Many folks, as they get into the hobby, tend to upgrade system size, so the fish for example have the habitat size increased in turn. But in the end, if a fish doesn't work out, because of size of system, or conflict with other tank-mates, or whatever reason, there is always the option to rehome him. So my opinion is always - give it a shot if it interests you, and just know your plan B in the event it doesn't work out.
It's written everywhere to not mix a Queen and Titan Trigger - as a rule. Here is my 7" Queen and 12" Titan living together in sin. There are no rights and wrongs when it comes to fish behavior, they're all individuals -