You can add another clown. You're right in thinking clowns don't get along...however, that's clowns of different species. Clowns of the same species get along great, and usually pair up. Depending on the species of clown you have, you'll want to add one of the same species that's either smaller or larger than the one you currently have. There doesn't have to be a large difference in size, but a visible difference is good. This will help insure they pair up. I wouldn't recommend adding a different species of clown though.
If your current clown is a Maroon, pairing them up is a little more difficult. You'll want to make sure that the second clown you add is much larger or tiny. If it's close to the same size, chances are, there will be a death.
You mentioned basslets...keep in mind that they can become quite aggressive so should be among the last fish added to a tank.
I'd also not recommend adding a tang, even yellows need a bit more space than a 55 offers.
Lawnmower Blennys are awesome, as are Starry Blennys and several others of the Blenny family. They might help with the hair algae. Unfortunately, not a lot of fish small enough for a 55 will do a lot of good. Rabbit fish and Tangs that do help with hair algae get a bit large for a 55. That being said, I do have a Spotted Rabbit in a 46, but it's small and I purchased it with the plans of it going into my 113 once it's set up and established.