You *CAN* make songs only with polychords (to answer my own question), the only problem is the approach with the MIDI Chord FX plugin... If you're lucky it sounds good (rarely), most times it may sound too dissonant...
I rather would encourage you to learn more about chords, for example with:
- "Music Theory for Computer Musicians" by Michael Hewitt
and
- "Harmony for Computer Musicians" by Michael Hewitt
Both books have helped me so much to learn more about harmony (and chords). That doesn't mean that I were the greatest composer out there but I'm still learning about chords and chord progressions and keys and modes etc. every day and in several years you may see Tricky Loops making classical Piano songs...
So I rather encourage you to learn more about chords and harmony than to use a MIDI Chord FX plugin by using "hit or miss"...
If you want to learn more about chords and listen to them simultaneously, ToneSpace may be a good practical solution:
http://www.mucoder.net/en/tonespace/