How would the ratios of iced coffee differ from regular coffee? If I'm making iced coffee, should the ratios of the ingredients (coffee, water, sweetener, milk) differ in any way?

If so, how?
Recipes vary according to taste, but if you want a similar concentration ("strength") of coffee, you'll probably want to use a stronger brew of coffee (basically amounts to using more grounds). This is because you're diluting the coffee when you add the ice (which melts), even more so if you're trying to start with freshly brewed and very hot coffee (melting more ice). Here's a suggestion from Chow and a discussion from Serious Eats that discuss the methods. 

Personally, though, I prefer the result of cold-brewed coffee. Rather than brewed-hot and served-cold, it's both brewed and served cold. I think the cold-brew extraction is a better complement to the cold service. Furthermore, less ice might be needed to cool the beverage to a similar temperature, which is an added bonus. See the myriad questions with cold-brew for more on that subject. 

As for milk/cream and/or sugar/sweetener, this seems to be entirely personal preference. I take mine black, so I'll let others comment on that.