I also have a ninety gallon tank. I have a large diamond goby who spends his time rearranging the sandbed and digging his cave. He is the most active of my fish and checks out every happening in the tank. I have a scooter blenny who goes everywhere in the aquarium but spends lots of his time with the diamond goby. I have a beautiful Midas Blenny but she spends the majority of time swimming in the center of the water with occaisonal visits to the surface to beg for food. And I have a red-streaked who is shyer than the other fish mentioned and he spends his time perching in various places. They all co-exist happily together.
The fish I did have problems with was a polypsedochromis, which is in the dottyback family. He is long in shape like the gobys and blennys. That fish pestered the gobys and blennies with his aggressiveness so I got rid of him. I traded him in for a nice ricordea rock.
It was a good trade IMO.
A lot of it comes down to the individual fish's personality and whether they feel safe. I have lots of rock with hiding places and I keep friendly fish so no-one has to watch out for a predator.