There really isn't much info on the Models 26 and 28 unfortunately, mainly due to the fact that they are not imported into the U.S. anymore. I'd like to know the answer to your questions as well.
If you go to Diana's web site they have a PDF file with their catalogue in it that you can download. The 26 and 28 seem to be the same power plant just different stocks. Velocities are 175m/s for .22 and 235m/s for .177 but they don't give a pellet weight but I would guess they are in the 10-12ft/lb range.