To me, you don't learn solos to learn scales, you learn scale patterns. While it's possible to learn scales from solos you'll never quite be able to understand them and it can take an awful long time (maybe for a few intelligent people it will work but for most it won't). If you want to learn scales learn scales, not solos, chords, etc... If you wanna learn to solo learn solos, etc...
If you want to teach improvisation you can have them learn specific solos so they can use them to get used to working the changes/playing in time/etc but if they ultimately want to be free to do their own thing they have to do it all. You shouldn't assign solos based on which scales they use but the complexity. Music isn't about scales and even if you knew the major scale perfectly doesn't mean your solos will sound any good.
If I were the "teacher" I would assign "solos" based on complexity, style, and specific issues it may solve for the student(if they student has issues with position changing I might try and find a solo for him that would help with it). I would also have the student work on scale patterns along side of everything else.
A student should be working on many things a little bit at a time and not one thing only. Scales, songs, chords, arps, solos, improvising, techniques, theory, etc... All these need to progress about the same although some are more important than others. This is assuming the student wants to do more than just play his favorite song.
Heres a list of the order of things that should be focused on by a beginner:
- Basic Chords (standard open position chords),
- Basic Scale pattern fragment(very simple 2 or 3 line box pattern),
- Simple fun songs for chord practice and maybe even a little simple soloing,
- Basic Theory (very basic as not to overwhelm the student),
- Possible sight reading
- Very simple techniques.
The lower stuff should be introduced subtly and without any requirement. Like if your teaching a pull-off you would do it very quickly but just mention a word about it when the context is right. e.g., if your teaching a simple box pattern you could say "You can pull off the note without picking if you want", etc...
Once they get more advanced you can then adapt the order for what they need. For instance if they pick up the chords very easy (which usually isn't the case but assuming) then you could focus more on scales and improvising and more complex chord forms.