Psssssh yes. I have 2 severums in my 125. I have a male jag, and a bigger oscar that I call the buffer fish (buffers the aggression down in the entire tank by being pretty passive and large).
My smallest severum is the most aggressive fish in the tank. Go figure. He wins every liplock with the jag and scares him away from his territory.
Jags are pretty passive overall. They almost remind me of my snook. Both are about 7-8 inches and very personable, but not really mean to the other fish in the tank at all.
A lot of times I see jags that are glass bangers, but I think its more tank aggressive (you know like dogs that are cage aggressive, aggressive to the things outside when they are in the cage because they want to protect their cage) since I have had fish that don;t bang hte glass, but are much more aggressive inside the tank (convicts, red devils, salvini, bumblebee aurutus, ah that GD aurutus!).
Definitely give it a try, just make sure its got some size, maybe at least half the size of the jag.