I believe you'll be okay in a 6'. Never homed one before since I only have a 55 gallon.
A very small one would be no issue, but they grow fast and need plenty of room in all three dimensions. I have a rowdy 2" clown tang (named Bozo, only fish with a name at my house) in my 90 gallon frag tank. He is happy there right now, but he will have to be moved within the next year or two at the most. They aren't the easiest tang to raise and they tend to be real fighters, so until he gets bigger, my other tangs would take him out.
A yellow tang is pretty much in the same category. When small he would be fine in your system. Look at the adult size of the fish and that will tend to lead you in the right direction. I even have this problem in my 300. Some fish are just going to get too big to be comfortable in there, so I will eventually move them to a friend that has a big fish only system.
No, it does. And the shape of the tank does to. The basic idea is that some fish swim great distances during the day and some rarely move 12" from where they were born. Some fish like tangs can get stressed (and then sick) if they don't have some open swimming space and room in the tank for tang sized caves and holes.
A 100 gallon is on the small side for most tangs, but with some careful layout a yellow tang, or a Kole tang would be okay. Hippos get big fast, but if you don't get too attached you can trade him in when that happens. My Hippo and various other tangs live in my 300 gallon. They as old as 10 years or more and fat. But honestly, I wish I had even a bigger tank for them. Believe it or not, the bigger the tank the stabler and easier to care for fish and coral in. The expense and maintenance work rises fast as well.
I have an engineer goby so I need to be pretty strategic about my rock placement.
If I were to lay it out correctly (could you help me figure it out?) could I have a yellow tang and mimic lemon peel tang in the 100 gallon?
Yeah. I understand that. That's how I feel. I've promised myself that I am going to get quality for this tank so I'm going to have to save up for the nice stuff. Lol.