Ordering anything I feel changes depending on where you go, but I would order a small coffee with 1 sugar and 2 milk as " I'll get a small coffee (it's implied hot unless otherwise specified) with 1 sugar and 2 milk. Thanks" Where I am from you generally have to say you want milk again after saying it the first time because coffee is almost always ordered with cream here. Ordering something like a coffee I find is usually done fairly casually it's normal to hear "Ill get a medium coffee, 2 cream, 1 sugar." I tend to be partially formal as to respect the cashier because service jobs like theirs can be rough at times and dealing with inconsiderate people all day takes it's toll. Basically in regards to ordering I do this: fast food = short and casual. Restaurant = longer and formal.
Oh also forgot to mention for some reason that I am unaware of when specifying the amount of sugar or cream you don't have to make the cream or sugar or milk plural. Example: "I'll have a large coffee with 3 creams and 2 sugars." Usually heard as "I'll have a large coffee with 3 cream, 2 sugar." Also just because we have gone to this much depth when ordering specific amounts some have there own slang names. Any coffee with 1 cream 1 sugar is a "single, single", 2 cream 2 sugar is a "double, double" and if you ever get more than that it just follows as triple, triple, and you shouldn't be ordering anything with 4 cream or 4 sugar :) you can also order using these terms "I'll have a small coffee, double cream, no sugar." I wouldn't mix and match though "I'll have a small coffee, 1 cream, double sugar." But I have heard it said that way. Basically there are a lot of ways to order, the best way is to just be clear, in my opinion.