If the I, IV and V(or V7) don't fit then try the relative minors IIm, IIIm,VIm or VII diminished.   So if you are in C and C, F and G7 don't suit then try also Dm and Em and Am and B diminished. I have attached an excel spreadsheet with all major scales and it provides all the relative minors for each scale. If you have EXCEL fine if not and you want this document then go and get the free software Open Office and use its spreadsheet function.

You might want to get Green's "Chord Chemistry" book. He does have advice on chord substitutions. But I am speaking beyond what I know.

I do not know exactly what you are doing, but is it possible that your backup tracks are already providing you the chords which you need to identify and the issue is to know what lead riff or what scales go along with these chords?

If this is the case you may want to get a demo version of Chord Alchemy 4.  For every chord it list scales that are suited to it. It also does a ton of other things including giving over 200 chord types, up the neck, in every key!

I hope this helps.

The important thing is to know keys and triads. If you can memorise this, then you've got it made!

Take the key of C major, no sharps and no flats. The notes, of course, are C D E F G A B. All chords are based on triads, using the 1st 3rd and 5th notes.

OK, try this. Take C as the root or 1. The 1st 3rd and 5th notes from there are C(1) E(3) and G (5) C major Do the same thing to the rest of the notes and you get this.

C E G = C major = I

D F A = D minor = ii

E G B = E minor = iii

F A C = F major = IV

G B D = G major = V

A C E = A minor = vii

B D F = B diminished = vii

If you have an A note in the key of C, the underlying chord can be D minor,  F major or A minor.

Hope this makes sense.