Welcome, John.
Please have a look at the
Help section of this forum. It's not complete, but will provide an overview of how things work and get you started.
Your database is completely independent of it's data sources. The program handles data from IMDb with little modification. You'll see in the plugin configuration there are a few options that require custom fields be added—if you want the optional data. All data plugins run in batch mode, and they can be run "silent"—doing the best they can to find the correct data without stopping to ask you for clarification. The way to update 400 movies is to run the plugin on all of them in "silent mode" (the option is at the bottom of
Preferences>Plugins). Then turn that option off, select the movies that failed to update (filter them with a search), and then run the plugin again.