Originally Posted by: evan.froeseMy question is this, if you want to improvise with a minor scale over a major chord progression, do/can you use A-minor and/or C-minor?[/quote]
In general you should use the relative minor scale. So, in this case, if your chord progression uses chords from the key of C major, then you would use the A minor scale. Because of course the notes of the C major scale are the same as the A minor scale (but numbered differently!). But, then it's better to conceptually think of it as C major, because of the scale degree numbers.

Work through my tutorials on improvisation. They're a great place to start to learn the basics.

https://www.guitartricks.com/tutorial.php?input=876
https://www.guitartricks.com/tutorial.php?input=483
https://www.guitartricks.com/tutorial.php?input=491
[QUOTE=evan.froese]
I'm guessing (with my admittedly limited understanding of theory) that you should only play A-minor scales over a C-major chord progression and a C-minor scale should only be played over a chord progression in Eb-major.

Yes, you've got the basic idea. When improvising, the general principle is to find the key of the song, then target chord tones as the chords change. Start by learning to stay in key. Use these 2 principles.

1. Use the notes from the scale that forms the key of the chord progression.

2. Target chord tones. This means to use the scale of the key you are in to melodically & rhythmically emphasize the notes of each chord as it happens.

Have a look at these tutorials on the basics of improvisation & learning to play in key over chord changes. Hope this helps. Please ask more if necessary. Best of success!