OK, those sound like good explainations...
The last "geek" explaination is closest to what I had in mind. Technical problems might have made sweeping beams easier to control/stabilize for the smaller ships. Though, I was kinda thinking that slash beams are older tech, with the larger, immobile beams being newer with improved tech.
I also had a different idea, regarding Shivan beam/reactor tech... Perhaps they use subspace tech to stabilize their beams, giving them that smooth look. This would result in a subspace signature in the beam, that adds damage by directly destabilizing atomic bonds at the target. The added stability means that they can fire their beams longer. The added damage means they don't have to use as high power beams as we do to achieve the same effect, and so they can fire more often. Added together, it means that, though probably more advanced than ours, Shivan reactors need not be THAT much more advanced. It's not always how MUCH you put out the front end, it's how you put it out that counts...
Well, that's my idea for Shivan tech:p