They can be. They can be aggressive towards each other and towards humans as well. Here's an article I wrote about one of my crested geckos who has, unfortunately passed away (I miss him):
Bart seems to think he's a flying gecko. Or perhaps he thinks he's an impresario. In a household of 65 or more reptiles, most of them geckos, it takes a large "personality" to stand out, and Bart certainly does. Every evening, the loudest sounds in the gecko room, also known as my living...