Hi Marie,
food combining has its origins from Dr William Howard Hay MD back in 1911, and Dr Herbert M Shelton as well some years later.
Lettuce and Celery combine with sweet fruits, OR sub-acid fruits, OR acid fruits, but these are the exceptions and not the norm' to combining any vegetables with any fruits. http://www.thewolfeclinic.com/foodcombining.html
A tomato is actually a fruit and not a vegetable as most people think for example, so you are quite right.