Absolutely you can use your Table-Service credit for a Quick-Service meal. You can not however trade in the opposite direction; a Quick-Service credit can not be used for a Table-Service meal. I recently used the Deluxe Dining Plan and we were able to use our Table-Service entitlements at Be Our Guest restaurant for lunch which is considered a Quick-Service restaurant during that time period.
I hope this has helped answer your question. If you think of anything else you may need assistance with please let us know. We love to help Guests have the most Magical time possible on vacation at the Walt Disney World Resort.