The last time I had people over who were omni, pesce and veggie, we had a roast salmon, a ton of vegetables and salad, and a (vegan by chance) vegetable shepherd's pie. (We also had cheese and pudding.) There was far more pie than enough for two vegetarians but an awful lot of it disappeared, it was being treated as a side dish by fish eaters.
policing individual servings is fucking weird
But presumably you would have said something if all the omnivores tucked into the vegetarian shepherd's pie as their main, leaving very little for the actual vegetarians? There was a whole thread on that recently (omnivores eating the vegetarian options on buffets, not realizing/caring that it left almost nothing for the vegetarians to eat).
If people have individual dietary requirements, you do have to 'police' manage servings, ie make sure X gets her vegan option, Y gets her gluten-free course.
My in-laws are vegetarian. When they're here, and we've got 10+ eating, I make sure the vegetarian main option which is large, but not large enough for 10 main portions when there's a meat dish, too doesn't get hoovered up before they reach it, and put aside non-duck-fat-roasted potatoes for them, etc.
I'm 'policing' because someone who's specifically told you they have dietary requirements is implicitly asking you to 'police': please ensure there's something that I can eat. I'm trying to provide a meal that suits them, but it goes both ways: you can't say you have a restricted diet, expect people to cater for you, and then act all '...what?' when it turns out that those restrictions only depend on your mood.