Adding A Pinch
The pinch is one of the basic ingredients of this style
in that it allows you to play a melody note at the same time as a bass note.
Once you have the one shown in the video (first beat of the bar) down you could branch out and try to get a pinch on any other thumb beat.
As I say in the video you could use your second finger to get the pinch rather than the third.
I have no rigid system of using a particular right hand finger for a specific string but quite often when I improvise in this style my 3rd finger will get the top string.
Some players play this style without using the 3rd finger at all and probably some players use the thumb and all four fingers.
I use the thumb to get the bass and whichever finger (out of 3) feels right to hit any particular note.