Is there a way to open tabs in the background? I don't want new tabs grabbing focus
I don't think you can have exactly what you want (without purchasing some tool with that capability), however, if your Finder preference for 'open folders in tabs instead of new windows' is checked, then you can open selected folders in new tabs with command-option-o. At least, you can in Sierra.
Note that it won't happen in the background as the last item selected will become the active tab in the window. However, your original selection will be maintained in the initial tab.