I have the walls open on my house and have discovered that a previous "electrician" has mix and matched 14 and 12 gauge wire within a single 15 amp circuit. I know that mixing in 14 gauge into a 20 amp circuit is a no-no.. . But is combining the wires on a 15 amp circuit OK?

Sure, no problem, #12 wire can be on a 15 amp circuit mixed with #14 wire. The danger is if someone sees #12 wire and ASSUMES it is a 20 amp circuit, and uses it or modifies it as a 20 amp circuit.

But you caught it and know it is a 15 amp circuit, so no damage done, and no codes are broken.