I would also go with a know juvenile. You clown after 1.5 years maybe a female. Add another female will be disaster in a 29 g tank. Even if it is not a female, since it is there for a long time, it will more likely to put up a huge fight in it's "home court". If you put in a smaller clown, it will capitulate quickly and there will not be a lot of bloodshed.
Percula or Ocellaris does not mater, they will pair up. However with Ocellaris and Percular cross pair, there is usually a bigger fight before they settle. I have a larger Snowflake and smaller Onyx percula, bother juviniles, together for about a year before the Onyx turn female. The Snowflake did not grow as fast as the smaller Onyx so after about 1 year the Onyx was bigger. There was a huge fight for 5 days before the matter settle. Th Snowflake fins were all rags and with multiple body wounds. He healed quickly and now I have about 500 SnowOnyx babies that I am trying to get out of my tanks.
Huge fight like this is unheard of if you raise Ocellaris or Percula pair together from juviniles.