I notice that although Pounds and Inches says not to mix vegetables, in Trudeau's book under Frequently Asked Questions (p. 130) it says that a mixed salad (which include onion, radishes, cucumber, spinach, and tomato) is the one exception to this rule. I'm wondering if any forum members are following Trudeau's book and how this has worked out for them.
I think some do follow Trudeau's ideas. Some with success some not. Its always a question as to why it was that Dr.S. says no mixing. There has got to be a reason. So, use your best judgement but, I wouldn't. Or at least try it once and see how it goes.
I have on occasion mixed veges for a salad when I just couldn't eat one more bowl of lettuce. I didn't notice a difference but I don't know that I would do it all the time. I am forever advocating that you listen to your body.
With that said I don't consider tomatoes a vege and feel quite comfortable mixing them with lettuce or cucumbers.