PoorBigBlue said:I personally wouldn't, but you might be able to get away with it. Some species of Watchman Gobys can get up to around 6" - the Yellow Watchman Gobys stay smaller, maxing at around 4" or so. But, with 2 clowns, you're already looking at 2 3-4" fish when they're full-grown, and that's already not leaving a ton of wiggle room in your tank. Add another one, and I'd say you're on the brink of being overstocked, if not over the edge a bit.
Then there's also the issue of the clowns already being established in the tank - they're likely to be territorial, and won't appreciate a new fish in their tank. They might bully the new comer to death, or at least force him into hiding. If you did add another fish, you'd probably have to remove the clowns from the tank, rescape, and then add all the fish at once.