Awesome to hear that you have decided to take an interest in Crenicichla. They are totally awesome, unique in just about everything they do. I have to tell you, once you start keepin them, you may become so addicted to them that you may not be able to stop. Keep in mind though, that they aren't for everyone. Some people just can't handle them, so if you can't don't feel too horribly bad.
Pikes CAN BE extremely conspecific, especially when they are adults. Thats why its best to keep them one of three ways, singly, in pairs, or in large groups. The 'in large groups' option requires large tanks, a lot of money, and a lot of patience. Not to mention tank dividers just in case.
They don't stake out HORRIBLY large territories, but the territories that they do stake out they are very protective of. In a large enough of a tank the issues with tankmates colliding is greatly reduced. Pikes are generally speakin aggressive fish, if a fish picks on them, they normally don't back down, even if they are the smaller fish. Driftwood, PVC pipe, clay pots, and all sorts of plants (fake or live), especially ones that are floating are great decorations and perfect for pike setups. Crenicichla generally go by the principal: 'Out of sight, out of mind'. If you have enough hiding places so they can get away from each other, then aggression is reduced quite a bit.
If you have a reasonably sized tank for the species, money, and patience you can get a mated pair of pikes. What you do is get a group of them while they are still young and grow them up together. Crenicichla don't like forced pairing, it can work, but its much more stressful on you and the pikes. I prefer allowin them to pair upon their own. A group of three is large enough, but your chances increase dramatically with the more you have. Don't get discouraged if they don't pair up though, as sometimes it just doesn't work out. It can be done, as I've done it several times with my pikes. Got them small, grew them up together, gotta pair. Its important though that once you get a pair you remove the other pikes of the same species that AREN'T in the pair because the pair will totally harass and try to kill the other pikes. Straight up, and a lot of the time they will succeed.
There are several groups of pikes: Lugubris, Saxatilis, dwarfs, Reticulatus, Acutirostris...and I think thats it...might be one or two more. Each group of pikes has its own special characteristics and traits. Lugubris group pikes go through at least one color change, most of the time two before they are adults. They also are the largest pikes, with Crenicichla sp. 'venezuela' growing to 24". When I say a color change, I mean a COLOR change. Crenicichla sp. 'xingu I' also known as the orange pike, are in the Lugubris group of pikes and are among some of the more aggressive species of pikes.
As they(xingu I) mature they will go through two color changes. First, they will loose the black bar and any other spangling on them, becoming one uniform color. This first color change is know as the 'teenage' stage. This is when they start becoming WAY more aggressive. Your once peaceful pike will start showing other tankmates whose the boss. Finally, once they are around 8 to 10" they will go through their last color change and get their adult coloration. If they are female, they will get a darker head and tail region, with their bellies bein burgandy to cherry red. They will have a WHITE stripe in their dorsal fin (males lack this). Males will be olive drab to brownish tan with very little to no spangling.
For most pikes LUGUBRIS group pikes it is rather difficult to sex them before they are mature because as juv. they look practically identical. There are a few other ways to sex them when they are young, but its not always accurate and takes a lot of experience. Males generally tend to grow faster than females, so if you get a group of them at around the same size and a couple of them shoot ahead of the others in growth, then they are probably males. Not always, but most of the time.
Saxatilis group pikes stay relatively smaller than Lugubris group pikes, max. out at about 12"...rarely get much larger than that. Most of the species don't go through a color change persay, but rather get more spangling and spots on their bodies. They do look amazing as adults, so if you don't like them as juv. just wait.... Females will get a cherry red to pink belly (this is a trait that almost ALL species of pikes exhibit...red belly/abdomen in the females), have a black spot in the dorsal, sometimes several spots, and in some species, have a white stripe in the dorsal as well. Again, males lack these traits. Saxatilis can be more aggressive towards other pikes though, but being more common gettin them in large groups doesn't cost a lot. Crenicichla sveni is a good example of a Saxatilis pike.
Reticulatus group pikes are like Saxatilis group pikes, except they have one very unique characteristic, an atrophied swimbladder. This makes them sit on the bottom a lot of the time, and when they do swim most of the time its awkward. Crenicichla sp. 'bellycrawler' is in this group.
The other two groups fall pretty much into one of these catagories so I'm just not gonna explain them. Alright, the reason those pikes at the LFS are being 'peaceful' with each other is because they are Lugubris group pikes. Lugubris group pikes are known to 'school' together when they are young for protection assumably. As they mature they start pairin up and leave the group. Saxatilis pikes do this to a certain extent, but not as much as Lugubris do. So just because they are swimmin peacefully together DOES NOT mean they are a pair. Keep that in mind. Aggression is varied between species. Some species are easier to keep together than others. Most pikes though DO NOT like other pikes, unless they are in a pair, and even then they can be aggressive towards each other.
Good beginner pike would be Crenicichla sveni, Crenicichla sp. 'belly crawler', Crenicichla johanna, and Crenicichla lucius. Most of them only reach about 12", with the exception of Crenicichla johanna which can reach 14". For the tank you stated to have, I would go with a Lugubris group pike, like Crenicichla johanna, Crenicichla sp. 'venezuela' (gets large), Crenicichla sp. 'xingu I', and Crenicichla strigata. Sp. 'venezuela' is one of the most common pikes, along with 'bellycrawler'...which was my first pike, loved him. Crenicichla sp. 'xingu I' are a bad choice for a beginner because they get large, are aggressive, and are aggressive. If you want that, go for it. They are hardy and totally awesome cichlids.
Hope this helped, if you have any more questions, feel free to ask! Cheers,
PP
Unless i have time to save up that is.