There should be a considerable size difference when selecting a mate. I believe they all start off as hermaphrodites (or males?) and begin to pair off, with the larger one becoming the female.
The one you have is likely already a female. Get the smallest healthy fish you can find and keep your eye on the little fella when you introduce him to the female,,, a bit of chasing and nipping is normal,, the male should respond by shaking in submission.
If the female tries to rip him to shreds then it probably ain't gonna work out.
And definitely check that they are the same species first... - I've heard that true and false percs can be put together, but IME, that is NOT the case at all.
All clownfish start off as sexless drones. The strongest one turns into a female and the next turns into a male. When the female dies, the male turns into the female and the next toughest drone turns into a male. I guess that means nemos "dad" was actually his female partner :lol:
What it means is that your clown is almost definately a female and therefore needs either a male or sexless clown in order to get along. If you introduce a female, one will probably end up dead. So choose the smallest one you can from a group and you should be right. Good luck.