okay, if a tune is in A minor (8A) it will more than likely mix well with other 8A tunes at the same tempo. Tunes in D minor (7A) will often also mix well with 8A tunes cos the D minor and A minor scales contain mostly the same notes (A natural minor scale - A B C D E F G A. D natural minor scale - D E F G A Bb C D), the same goes for tunes in E minor as the E natural minor scale also contains mostly the same notes as A minor (E natural minor scale - E Fsharp G A B C D E) A minor tunes should also harmonize well with C major tunes as A natural minor and C major scales contain exactly the same notes just with a different root.
so to sum up, if you look at the chemical key chart and the tune is for example 5A, it will (in theory) mix best with tunes in 5A, 6A, 4A & 5B. It should also harmonize well with 6B and 4B as they will contain the same notes as tunes in 6A and 4A respectively just with different root notes.
sorry if that was confusing, hope it helps