We just added a small perc with my larger perc. The larger one terrorized the little one. (no doubt in my mind the big one would have killed the little one...).
So, I came here to find out how to make them get along. (you can search and find out what they told me to do)
this is what i suggest doing. put the one you have in a bag, acclemate the new one, keep lights off, move lr around (so the one you have will lose it's territory) then relase the percs, cover the tank so it's dark, be careful the heat doesn't get too high though.
You may not need to do that, that's what we had to do, but I have a mean clown... Bagging the one I had was the only way to save the little one, we let the little free in the tank over night with the one we had in the bag, next morning let the bigger one free, and covered the tanks. they get along now, the little one follows the big one around which tends to agervate the big one.