Ok i could use someones help... in the theory lesson lesson, it says he makes a solo using major scale + triads.

So i was wondering... what keys do the triads fit into?

Its pretty obvious you use major triads with major scale and minor triads with minor scale as i see it...

But what about diminished/suspended triads? Are they only used with certain modes, or can they be used with the major/minor scale or what?

thanks in advance
an augmented triad can be the III chord of the harmonic minor scale which is the same as natural minor (Aeolian) but has the leading tone or a raised 7th degree this changes the III,V,and VII chord qualities
Triads, I'd say are very useful and can add lots of interest to a solo. One way to look
at them, from a guitar pattern perspective, is that you can do a 3 string sweep where
ever you happen to be on the neck.

I think your confused about what "keys" the triads need to fit into. In any scale
you can form a triad from ANY note in the scale by harmonizing it to the scale. For
example, in the major scale, you can start a triad out from ANY note you happen to
be on and it will give you either a major, minor or diminished triad. You're pretty much
free to use any of those and you'll still be "in key". Other scales (other than diatonic)
will have different collections of harmonized triads.

A really good practice study is to do scale triads moving vertically and horizontally
on the entire neck. It can be a good sweeping practice plus it can really help you
see the whole neck better and be able to find a triad no matter where you are. Very
nice stuff for solos.