Why are my fire mouths going after my peacocks?

I have heard that its one or the other with firemouths: either they are mean as Hell, or they are big wussies. My guess it that your mixing SA and Africans, they aren't really made to mix. The two communicate differently. For example: my geophagus follows my large front around doing his little spaz attack dance saying 'I'm the big guy around here" and my front just goes about his business. I would seperate the two types if possible.

Another possibility could be just that they are getting to 'that age'

+1 on totally different part of the world, South American cichlid species aren't like Africans, S/A species will bully each other to establish a pecking order for the tank, they will do this because it's natural for them. They do in the wild as well, it's how they show their dominance, to show "everyone who's the big bully".

Flaring of the gills is to let others know they aren't welcome in that area, and if they stay in the area the dominant fish will then attack.

Your best bet for a peacock tank is a 75 or larger, most species need more space than a 55 when they're adults. As already mentioned above, mixing SA or CA cichlids with African rift lake species isn't a good idea. Aside from the difference in water conditions their behavior is also completely different. Firemouths in particular claim a relatively large territory when they pair off, and will consider the whole tank as thier own unless a more dominant fish takes over. Even then the dominant fish may get torn up or killed if the firemouths pair up and spawn.