The Disney Dining Plan brochure can be downloaded from the WDW website.
This includes a listing of all restaurants that are on the Dining Plan,
as well as additional information about number of credits used, counter
service vs. table service, etc.
To download the brochure, click
HERE, then click on the Disney Dining Plan card, then click on "Download Disney Dining Plan PDF."
Unfortunately, there is no option to pay for part of a two-credit meal with dining credits and part with cash. The posted menus at the counter service restaurants do have indications about which items count for meals and snacks on the Disney Dining Plan. At table service restaurants, there are very few exclusions on the Plan, you will be able to order most items on the menu. When you are seated, you will be asked if you are on the Dining Plan. If you are, your server will give you all the information you need to order. If you have questions, they're happy to help.
Enjoy your visit to WDW.