There is actually a difference between mixing greens with fruit, and mixing fruits and vegetables. Greens are closer to what we consider leaves and grasses, than they are to being a vegetable.
What is actually a vegetable, are mostly what we consider a root vegetable, like a potato, carrots, turnip, or beets. Some other examples that are not root vegetables might be things like broccoli, cauliflower, or Brussels sprouts.
Many of the foods that we consider to be a vegetable, such as tomatoes and cucumbers, green peppers, or squashes, are actually the fruit of that plant, so they are really a fruit and not a vegetable.
Food combining says that we should not mix fruits and vegetables, but we often do, when we have meals with a vegetable , and then maybe some kind of fruit for dessert.
Mixing either a fruit or a vegetable with greens is perfectly fine, so making something like a green smoothie with fruit and greens works out great.
One of my favorite juice drinks is fresh carrot and pineapple juice, which does involve mixing fruit and vegetables.
I do think that when we are making a juice or a blended drink, it can digest faster; so it is okay to mix in some vegetables with the fruit, but it probably depends with each person, and how strong their digestive system is.