Can I use an instrument other than guitar with my Play Acoustic / Electric to trigger harmonies
Yes, the Play Acoustic / Electric can also be used with a most chordal instruments to trigger it's harmonies. In this case you'll want to make sure the signal being plugged into the Play Acoustic / Electric is a direct feed from the instrument, it would be best that the pickup is not microphone based so stage bleed is not an issue.
You CAN NOT use a microphone directly into the instrument input since the input on these units are design to handle instrument and most line level signals only.
Ultimately, this is something that you will need to test with the instrument you are using, to confirm it will track the chord changes and give you what you require.