On a related note: Cucumbers in fruit salad are great for moderating sweetness (or the bitterness of grapefruits). Other veg I like to put in fruit salad include lightly sweet veg like carrot and sweet potato.
I've had "vegetable" salads with apples, pears, grapes, nectarines (not all together lol), a bit of sweet-tartness works really well, especially with a vinaigrette type dressing.
Tomatoes are a vegetable (fite me bro). They're botanically a fruit, but they're high in acid and don't play well with sweet fruits - Avocado is a good other example that goes terribly with sweet fruits especially citrus.
That said - avocado salad (aka guacamole) is amazing and tomato salad ditto - whether just tomatoes with salt or caprese.
It's not that tomatoes can't be used in fruit salad it's that they pair terribly with Honeydew, Grapefruit, Orange, Grapes, and Cantelope.
Several years ago my wife grew over a dozen styles of heirloom tomatoes. There were several that I think would have worked in a fruit salad, particularly with watermelon and some basil.
Yeah, chilled, and with some aged balsamic. I also enjoyed them with the balsamic, fresh basil (and some mint), some olive oil, chopped green onions, and cucumbers for another good salad.