You cannot combine tv show episodes and movies in the same library.
You could create a library type ‘Other Videos’ - but then you won’t get any metadata automatically.
The best way would be to create a ‘dumb’ playlist.
If you are using a ‘filter’ in your library, make sure to ‘select’ the items before you click on ‘Add to Playlist’ - otherwise you’d be creating a ‘smart’ playlist which cannot work across library boundaries.