By intervals I am assuming you mean the pattern between notes as it relates to the scale for the chord being played over...yes?Agreed, I've never been on board with the "Only play X string instruments" advice. Notes are notes. It should be no problem for someone to switch between basses with different numbers of strings.
One thing I'd add to drilling where the notes are is to drill intervals in relation to those notes. If you're soloing you're probably not going to play the root to start or end a phrase but you will still probably use it mentally as a reference point for where you start or end a line.