Pluto is a planet, though. It’s officially considered a “dwarf” planet, and as “dwarf” is just an adjective, it’s still a planet (just like a short person is still a person). The other 8 new dwarf planets (Ceres, Eris, Makemake, Haumea, Gonggong, Quaoar, Orcus, and Sedna) are also all planets - so we have 17 planets total.
Seriously, though. By the same 3 criteria that Pluto isn’t a planet, Mercury isn’t (as it isn’t in hydrostatic equilibrium).
If you’re talking about tomatoes, the difference is the context, and it isn’t a choice between colloquial vs scientific taxonomy, but between culinary/nutritional vs botany/taxonomy (and). You can talk about either in a colloquial context or a formal context, though generally there isn’t much reason to talk about botany in a colloquial setting.
From a nutritional perspective, mushrooms are generally considered vegetables, too.
I thought you were wrong but I looked it up and I appear to have been mistaken. It makes “tomatoes are fruits, not vegetables” sound nonsensical, as it implies that “vegetable” is a different taxonomical option, when really it’s just a word for objects with a particular collection of traits that are relevant in a different context. What we should he saying is “While tomatoes are not fruit in the food pyramid, taxonomically, they are.” Doesn’t really roll off the tongue, though. Maybe “Tomatoes are vegetables AND fruits!” would solve that?