Try and keep as many of the chords as you can DIATONIC (to the Key you are in). If you need to make a ‘passing chord’ or one which has a ‘chomatic alteration’ in the melody :-
Use 1st or 2nd Inversion chords (eg. In the Key of C Major - B/D#, E/G#, A/E, F#/C#).
And/or Diminished chords (of which you know all about these already) 
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HAYDN is the man to listen to. Check out his string writing in his Symphonies. That would be an excellent strating point dude.