Are model/row actions done on a without sharing" basis?"

I want to create my own internal messaging system using a custom object that is designed for my users’ specific needs. I want the messaging system to work across all users of the org, but I cannot allow custom sharing of users or making the system default public for users. Without those settings, subordinate users cannot assign records to superiors in role hierarchy or to users of different branches because the superior users and users of other branches are not available as options in the lookup fields. What I am thinking of doing is creating an additional custom object called “directory” and having basic information on each user as records in the directory. I would make this object public. This info would include the User ID. I could then populate the User ID using actions and use it to share the record with the superior, but only if actions run “without sharing”, meaning that the sharing rules are suspended and the action executes regardless of sharing entitlements. Anyone know if this is the case with Skuid? I think it is the default for the API and Apex, but it can be forced into “with sharing”, so I’m not sure which Skuid uses.