Thankz stotty, got lots of great pike photos in there for sure!! There are SOO MANY MORE types of pikes out there though than are shown in that thread though!! How kewl is that? There are still new pikes being found and more still yet to be discovered!! Thats an awesome prospect to think about!
Now, about their aggression. Yes Crenicichla are aggressive. HOW aggressive they are all depends on the species, and ultimately, that individual fish. Pike cichlids have very varied personalities, some pikes will be friendly and bold, where as others from the same species will be aggressive or shy. Thats one of the attributes that makes them so addictive to keep, along with their unique shape, predatory nature, and beautiful colors. Generally speaking, pikes from the Lugubris group are gonna be more aggressive than ones from the Saxatilis group. There are more groups, but those are the ones most commonly seen for sale. For instance, Crenicichla sp. 'xingu I' is one of the most aggressive pikes available, where as Crenicichla sveni is more subdued and way more less aggressive than xingu I.
Be ready for aggression. I've had to keep pikes solo because they wouldn't get along with any other fish, including plecos. I've also had pikes that were completely community fish and i had no worries about. Just plan accordingly and don't do stupid things like keepin them with tetras, as they will ultimately be eaten. Use good judgement when pickin tank mates.