Picking fish is going to be highly dependent on bioload, tank dimensions, and the different types of fish that will have to coexist. Like for example, you're not going to be adding a tang to a 60g due to the small dimensions (lack of swimming space) and high bioload for a tank that size.
What I learned from stocking my 150g is that there is no set rules to abide by and most of it depends on common sense. I have two zebrasomas that will never exceed 8" for tangs (they're about 4-5"). When I started the 150g, I was told I could keep tangs by many people and if you read online, it's not recommended for a four foot tank, so you really gotta use common sense here. My yellow and purple have ample room to swim, but I'd never put in a 1 1/2 foot naso tang in my tank.
When I started stocking wrasses, I was told I could only have 3-4 wrasses total in a four foot tank, yet I have eight smaller wrasses and they all get along. So it all varies...
