All very good info. With the clown fish, you have to get an Anemone that is about 3 times the size of the clown. Clown's "buffer" the Anemone non-stop to secrete a chemical from there body that repel's the sting, or "buffer's" it. This action will kill an Anemone if it's not large enough. You did the right thing by taking it back. Your tank wasn't ready for that just yet.
Maroon Clown's are aggressive fish, mine RULE's my 225. Up until I was dump with a Bristle tooth tang anyway. The Tang & Clown have been fighting, the Tang is bigger and was winning ( cut the clown good once) chasing the Clown back into the Anemone. But that's over now, D.french has the tang.
Trust me he's not lonely. He will feel intimidated when you add another fish. He'd prefer to be the only one there. Keep in mind Maroon Clown's can get up to 8 inches when fully grown. I've seen them fully grown and they are HUGE and very THICK bodied. And you don't need to buy the Clown and the anemone at the same time. You can wait until your tank is ready, then buy a large anemone and the clown will host to it. It's really neat IMO, mine clown take's food to it all the time. And hide's in it at night. But anemone's can be nasty, as like what Yote explained and like Biff stated. IF it die's and your skimmer isn't large & in charge the death on an anemone can wife out your tank. And clown's will host in almost any other coral.