This thread might be better in the reloading section but in the meantime. I agree with you that you should start low and work up to these loads but to be honest I would match these loads against data from known sources, such as loading manuals or powder manufacturers web sites because as far as I am concerned these loads are just someone's guess.
As far as unsuitability for the X rifle, barrel length will really only affect one thing and that is muzzle velocity. The website info is actually tought out well enough to state the barrel lenght and other components used, which I think reflects well on the writer of that data. This way he cautions you that he reached these velocities with a 26 inch barrel, something that you will probably not match with a 22 inch barrel.