I recently learned about key signatures, circle of fifths, etc.

My question is: If I want to play the chords in a jam session, rather than soloing through a scale, does it matter if the person soloing is using a pentatonic versus seven-note scale?

Like if one player is jamming in key of C, using the major pentatonic scale, does the person playing backing chords omit the subdominant and the subtonic from his/her bag of chords?

Or does natural major versus major pentatonic ONLY matter for the person playing individual notes?

As far as keys and soloing are concerned the pentatonic scale IS the major or minor equivalent - you're just not using all the notes. Likewise any chords will be derived from the full 7 note scale, not the pentatonic counterpart.
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There's no reason to change what the backing is doing besides you're getting it the wrong way around. The backing dictates what you do as a soloist and not the other way around.
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Basically, what the other guys said. The chords determine what scales to play, not the other way around. And playing the major pentatonic scale is just playing the major scale without the 4th and 7th interval. You certainly wouldn't be hurting anything by not playing those notes just because the chords might be playing them.
Okay. I thought maybe the same way sticking to major pentatonic notes in a solo gave a certain feel, that the person playing chords might need to similarly stick to those five chords playing a major pentatonic chord progression, not a natural major chord progression. Now I think I understand better.

So basically there is only one major chord family, you can play any of the seven chords (or more if you know when to make exceptions) whether the soloist is playing notes from the major scale, from the major pentatonic scale or even a minor scale that "fits" with that major key.

Is there also just one minor chord family, not a different chord family for the natural minor versus harmonic minor versus melodic minor? So if you knowa song is in the key of C minor, you then know what chords are at your disposal, but the person soloing can then choose to solo within the natural minor scale, harmonic minor scale, melodic minor scale, or even perhaps a major scale that "fits" that minor scale, depending on the feel the players are going for?

This would make life a lot simpler than I had envisioned, only really having to learn 24 sets of chord groupings, one for each major key and one for each minor key. I thought there might be different chords you need to know to use depending on whether the song is in harmonic minor, melodic minor, natural minor, etc.


p.s. Thanks for feedback, too.