The problem with x-ray lasers or gamma lasers is that IRL we do not know how to make a lazing chamber which lazes in either of those spectrums. however, if this was an option, they would likely basically result in a laser which exploded on impact, as when a laser hits something, it throws up a 'cloud' of dust which then reduces the effectiveness of a solid-beam laser, however when using X-rays or gamma rays, there would be so much more energy in the waves that they would cause extremely fast heating of the target, resulting in what appears to be an explosion.
in regards to mirrors: there should be a different mirror block for each range of waves, and perhaps a much more expensive mirror which could change its reflectivity to reflect a larger range of waves
Perhaps in the production of the laser, you could make many different lazing chambers, which are interchangeable but each produces a different color of light
also there should be a chemical laser, which uses some sort of fuel (http://en.wikipedia.org/wiki/Chemical_laser), and a electrical laser which consumes a large amount of energy, but which can be charged(http://en.wikipedia.org/wiki/Free-electron_laser)
also, not sure if this should be here or in the gregtech area, but maybe add a laser which is designed to result in fusion, so there is an alternative to the tokamak, but which must be cooled substantially(http://en.wikipedia.org/wiki/Inertial_confinement_fusion)
lastly, cooling the laser is a big problem with lasers, so maybe to nerf them a bit you would be forced to use coolant, (liquid air for low tier, then liquid nitrogen for mid tier, then liquid helium for high tier)