I've had three kitties, and none of them has liked salmon, but if your kitty does, I don't see why not, as an occasional treat. It certainly can't be any worse than my giving my kitties bacon...
If it's easier for you, being a vegetarian, you could try dehydrated raw kitty treats rather than buying meat. Chicken, tuna, beef, salmon, whitefish - there are a lot of different flavors. Most of them you can either give as a dry treat or crumble and use as a topper on other food.
Honestly, I have no idea what sweet corn is as opposed to grain corn. I'm allergic to corn and didn't know there were different kinds.
But personally, I'd probably say no, no, and no, to both kinds of corn, polenta, and couscous. Kitties aren't built to digest carbohydrates, and feeding carbs will tend to make for fat kitties. Plus grains are a common allergen, which is why many people try to feed grain-free foods.
My kitty likes cheese too, and some kitties like yogurt. But again, if you have a kitty who's lactose-intolerant, giving milk products might result in digestive issues.
eldercat, I've been thinking about giving egg to Celia, since she's anemic. But eggs are another thing I don't eat, so I've been hemming and hawing about how to cook one. I keep buying dozens of organic eggs and giving them away, lol. One of these days I'll work up the courage and hard boil one.