Key of Song

If a song is in the Key of A minor or C major, I use the C major/A minor scale to solo along to. Because the Major and Minor scales are simply modes of one another- they the the same patterns on the fret board but with different root notes.
So if songs are in the key of Major or minor.. How and when do you get songs in the key of A blues.. or C harmonic minor?
Im about to learn a whole lot of new scales but dont know how to incorporate them into songs.
As far as i can see every Guns n Roses and Aerosmith song uses just major or minor scales!!
Take knockin on heavens door, G D C, Is it possible to play another scale over these chords? like the harmonic minor scale? (ive heard its good)!

If it is happy, then it is major, if it is sad then it is minor! :D
Well that was a stupid joke

Knockin on heavens door is not just major. After the second solo it goes G D C Bb and it is G minor

I think that when you are in a certain key you can use all the relative modes depending on what you want to acheive.

Chords or Arpeggios & Scales are relative to each other , or you can say the rhythm chords are made of notes from the solo scale or vice versa.

The chord progression is what mainly determines which scales should you use ... Some chord progressions offers you wider range of scales , other do not.

i.e. If you have a prog like : Am , Dm , Am ... It resolves into an Am/Cmaj key , so normaly you'd go to play A minor (aeolian) mode but you can still play A harmonic minor over it too.

If you added a G#maj for example , you wouldn't play A minor since it doesn't include a G# , and you'd probably stick with the A harmonic minor instead since it has the #7th G#.

Usualy , as you increase the number of different chords in your progression , you narrow the choices of using different scales .

If you play an Amin chord only ... You can play A aeolian , phrygian , harmonic minor , pentatonic , or any minor scale that resolve into A.

Anyway, don't lock yourself into these , you can always include some accidental notes and dissonance as long as it appeals to you.
Your ear is your judge.

Harmonic Minor - If your playing in the key of A minor and the chord progression contains a dominant7 chord built on the 5th scale degree (E7) you are in Harmonic minor.

Blues Scale - this scale is a little different then your major/minor/modal scales in that chords are not constructed from this scale. Remember the western/european major/minor system is thousands of years old, but the Blues is only a couple of hundred years old, so traditional theory does adhere to the blues scale phenomenon.

Traditionally this scale was constructed to be used over a 1 4 5 progression of dominants. Ex; key of A {A7 D7 E7}. However you will find that this scale will work over both major and minor progressions (A D E) or (A- D- E-) or (A- D- E7).
The historical importance of this scale was the usage of minor based melodies overtop of major sounding chord progressions.