In our setup, we have saved a recipe called 3rd_party_model_import.dip using the Import Profile editor tool (look in Tools/Utilities ribbon).
This recipe specifies (among many other things) that a template model should be used when importing parts and assemblies.
Also, we have created template models and assemblies and configured them with metric units and the typical parameters, key layers and datums we use in our regular models.
Then in the config.pro, we have the line:
We typically import in STEP files; with this default recipe in place, they always come in with metric units (the size conversion happens automatically).
We use import profiles here. Also we have mapkeys to automate further.
We set config option intf_in_use_template_models to yes, although not necessary when using profiles or mapkeys.