From what Pure has told me Gibby's are territorial. They need to have space enough and then a smaller Pleco can be added to the tank. I just introduced my Tiger Pleco and they are still figuring each other out, territories and all. But I havent had any fights. Here's the pm reply from him:
"As long as you have room...sure. The thing is, they are not aggresive, but more territorial, and in most tanks a plec the size of a gibby there isn't enough room for another large one. How ever they normally do ok with smaller plecs.
Example:
http://i4.photobucket.com/albums/y12...clownplec2.jpg My royal and clown hang out together all the time. But I dare not put another large royal in there. That I'm affraid would end very badly.
The key to keeping multiple plecs is A. have enough room. B. Make sure you have more than enough caves for everyone. And I do stress MORE than enough."