Well, when going by liveaquaria.com's suggestions, gobies may attack other gobies in small tanks. Maybe if you had a mated green clown goby/watchman goby? Not sure. I'm sure someone will chime in :)
They should be just fine. I have a watchman goby and green clown goby in my 29g tank. Mine get along good. In face I don't think they even see each other.
Go for it. Those clown gobies can take good care of themselves, and it will probably establish itself in a different area of the tank than the watchman (upper tank vs. sandbed).