Can I substitute milk or cream for half and half in cooking?

Yes you can but if you have both, you can mix them to make half and half if you want. When i make cream soups, I use light cream instead of heavy.

No problem for cooking, be cautious about adjustments when baking.

Yes, it won’t be as thick or rich but it will work if you don’t care about that, or if you have something else to make it thicker (corn starch or flour, depending on what it is you’re making). Regular milk will be better than 2% or fat-free. Regular milk will be closer to how the recipe is intended to be.

If I really need to lower the fat (like in Mac and Cheese which is really high in fat already) I use evaporated skim milk.
Otherwise, yeah, I have used half and half or even sometime whole milk instead of cream.