I just finished installing ST9 and upgrading to windows 10. No problems in general.
One of the things I customized were the template CAD files, one each for Draft, Assy, Part, and sheet files. I don't use weldments yet.
But when I start an assembly from a file (as the first part of that assembly), the program uses the ANSI standard rather than the one I have set up to be used when starting an assembly from scratch.
So here is the question:
How to I point the "assembly of active model" command to also use the same template file that is used when creating an assembly from scratch?
If you click the 'Browse' button on the Create Assembly dialog it will bring up the New File dialog where you can choose a new template. It will then remember the template you selected the next time you run the command. Is that what you are looking for?