Hi Agatha!
To address your questions:
1). If I understood correctly, then you can try this: go to “Appearance -> Editor” and add the following line at the end of the “style.cs” file (should be opened by default):
#portfolio-list li.portfolio-item { display: none; }
and do not forget to hit “Update File” to save the changes ;)
2). Fuji does not have built-in support for video thumbnails yet, but you can install any of the WordPress plugins that do the job, e.g. this one, which I often use myself. For a sidebar widget, you can try e.g. this plugin.
Best,
Andrew | Satori