Having a "centerpiece" fish isn't a bad idea. If it would be a Tang, a Kole Tang is a good choice since they are small at maturity by Tang-standards and for a Tang you are kind of at the smaller end of tank-size. They're also, from what I've read, Koles are less likely to be, at some point, nasty and aggressive with tankmates. On the other hand a "centerpiece" is usually a more highly colored fish than a Kole Tang is, but it's a matter of taste. I have a Chevron Tang in my reef and it's only moderately aggressive but it's still fairly young. It's growing like wildfire, though, and I fear that eventually I'll have to get rid of it the way I did with a Striatus Tang. After that...no more Tangs for me.
For an herbivore, you could rely on snails and hermits or even add a Lawnmower or Bicolor Blenny.
I don't think having a Royal Gramma and a Fridmanni together is a problem. A 75 or 90-gallon tank should be big enough at 4-feet long to provide sufficient room, particularly if there's a good amount of live rock. I have both and even when they come across one another, within inches, they ignore each other completely.