Hi Michal, I play some fingerstyle acoustic stuff and my view is that learning to play scales in the standard positions (like Am pentatonic starting on string 6, fret 5 or G major scale starting on string 6, fret 3), is not very useful for the music that Iām playing. Thatās because when playing fingerstyle (Iām thinking mostly Travis picking), you have to keep the bass going and canāt start improvising up on frets 5ā8 (where you canāt play the bass notes).
Whatās more useful is being aware of the different notes in a scale (could be major, minor pentatonic or major pentatonic) available when you are fingering a given chord in the open position. As an example, say you are playing Freight Train (which I know you have learned). That is in the key of C, and itās useful to finger a C chord and know the notes in the chord and also the other notes in the C major scale that you can play with your free fingers: G (string 1, fret 3), F (fret 1), D (string 2, fret 3), B (open), A (string 3, fret 2), etc. A good exercise is to pick out the notes of the C major scale while holding down the C chord. Even better (but much harder!) is to pick out the C major scale while playing a Travis style bass line. So, Iād say the video that you posted is not too relevant for this kind of music.
You can also do this with the other chords in the song: G, F and E (in the case of Freight Train).
For other keys, you will learn the notes for other scales. Also, many times in blues songs youāll be focusing on the minor pentatonic, instead of the major scale, or perhaps taking notes from both.
BTW, learning how to play scales in open position while fingering chords is useful for improvising fingerstyle, but itās pretty advanced. At your level (and mine, too, to be honest), I think learning a bunch of fingerstyle songs is more important than learning how to improvise. Itās nice to understand where the melody lines come from, but itās not really necessary to know the scales to learn the songs, imo.
Now I touched slapping technique. 
