I've always considered tomato a vegetable, because it is often treated/cooked as a vegetable. (It was a big joke when President Reagan said ketchup could be counted as vegetable in school lunches.)
I just looked it up again and see that your statement is correct:
People also put things like mandarin oranges in salads (with lettuce), or puts nuts (legumes) in vegetables dishes. Even having further clarified, for myself, that tomato is a fruit (and yet treated as a vegetable), I don't think there are any rules about whether to mix fruits with vegetables (or anything else) if they taste good together (and particularly if both foods offer a different type of nutrition).