The instructions with this light, leave something to be desired. Can I use the Glock adapter which is preinstalled, on my Sig P226? There are some other adapters in the box, but I cannot figure out if they would be preferred. Thanks

Yes.

The standard 226 rail is not 1913 spec and will not work with that adapter. The Glock slot is the right one.

I'm not up to speed on streamights as I use surefire. But the exact model of 226 does matter for which rail you have and on an X300 anyway makes a difference. For example M25's have a real 1913 rail as do combats and maybe other models (I am not an expert on this area, but I do own those two).