You can definitely add another clown, but I would avoid adding one of a similar size. Clowns are hermaphroditic. When they are of a similar size and they pair up, they have to decide who is going to turn into the dominant female, and who is going to stay the submissive male. This dispute over dominance can turn into fighting, and can lead to the clowns injuring each other.
If you have two clowns of different sizes, they already know who is the bigger, dominant one, so they won't fight over it. The larger one becomes the female by default, and the little one stays a male.
So when you get your second clown, try to find one that is either much larger or much smaller than your existing clown. That won't give you a 100% chance that they won't fight, but it will greatly reduce the friction between the two fish and increase the odds that they will get along with each other.
Also, if you have a percula now, stick with another percula. Different types of clowns will normally fight with each other, sometimes to the death.