I've read that it's OK to give our parrots the occasion dairy treat (cheese or yogurt) and that it's OK to cook with milk. I've also read that only certain kinds of cheese are OK (a hard cheese like Cheddar would be OK while a soft cheese like mozzarella might not be). And then I've read that we are to never, ever give dairy to our parrots. I've read third-hand accounts about parrots needing surgery to remove undigested dairy from crops and digestive tracts (I have yet to speak to an avian vet who has had to do this, though).
Now, parrots are unlikely to encounter dairy in their native environments unless they happen upon someone's discarded cheese sandwich. They don't need it to survive and likely shouldn't have it.
That said, I do indulge Pickle and give him a wee bit of regular Cheddar or a couple of licks of yogurt (organic, never with artificial sweeteners). Not every day, of course. Yogurt maybe a couple of times a week and Cheddar whenever he's going to or coming back from being to the vet, groomers or boarder.
I can't imagine that a small nibble of pizza would hurt your flock so long as the toppings or sauce didn't include any alliums (onions, garlic, shallots, leeks, chives, dill, etc.). There are those that believe that human food or "junk food" should be verboten but quite honestly, I don't feel that the occasional nibble of human foods would be harmful, either immediately or long-term.
In regards specifically to the chives: I simply do not, under ANY circumstance, feed ANY member of the allium family (onions, garlic, shallots, leeks, chives, etc.). I doubt that the random ingestion of small amount would hurt but I'd rather not take the chance.
Leslie Moran, who does quite a lot of writing and research in regards to parrot nutrition and health, has addressed both dairy and alliums in recent issues of Parrots magazine:
As far as beet tops, I don't know if they are OK (I've never heard that they weren't) and in regards to spinach, I don't know. I wouldn't feed it every day. If you make mixes or mashes ahead of time, maybe alternate weeks in which spinach is included.