What can I say at first-food restaurants?

I might have to ask this in the Q&A section. But, please correct the followings. Also, please give me comments if possible.
The followings examples are just for natural conversation usage, not for perfect grammars.
So, I can't learn them through textbooks.
I'd like to know what native English speakers say at first-food restaurants like McDonald's.

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(Just after "How may I help you?" by a sales clerk) Excuse me (Too polite?, simply Hello or Hi? of course, saying nothing would be a choice).

You can say "Hi", or maybe just "Yes" as a lead in to your order.

"Yes, I'd like xxx to go, please."

Can (May) I get XXX? For here.
Can I get XXX for here?
[Is "a" needed in front of XXX in the case for a countable food?]

Yes, "a" is need in front of countable food.

"I'd like a burger and a small fries." (Note that saying "a small fries" isn't technically grammatically correct, but it's widely used and no one will correct you. It's basically a short form of "a small box/pack/set of fries."

You don't even have to say "for here", you can wait for them to ask "to go or for here?" or "take away?" and answer accordingly. I usually forget, so they always have to ask me. (Whoops!)

May I have XXX with the (its?) meal?

May I have the meal of XXX?

May I have XXX? Its (the?) meal.
[Q: What should I say when I want a meal together? Salesclerks will ask about that later, but I want to say it in advance]

Do you mean when you want to specify the contents? Like: "I'd like the Big Mac meal with a coke, and large fries." That should be fine in that case.

From your comment: If you ask for a Big Mac, but want to have a meal/set, they will usually ask "Anything else?" or "As a set?" and then ou can say "Oh sorry, the XXX meal/set." 

Could I have XXX? I'll have (it?) here [instead of "for here"].
[Q: Is "Could I" too polite at first-food restaurants?]

I don't think "Could I" is too polite, but mayber superfluous. If you're ordering a normal menu item, you can say "I'll have...." "I'd like...." and then if you want to follow up with a specific request "Could I have it to go?" or "Could I have it with extra barbeque sauce?" then you can use it. 

"I'll have it here" is fine.

I'll have XXX.

I'd like XXX.

XXX, please. [Q: Too casual?]

All of the above are great! :D

Please give me XXX.

This one I'd probably not use, it's a little bit demanding. Using the "give me" makes it become more ofa  direct order. "XXXX, please" is better.

ANYTHING ELSE?

A lot of the fast food/restaurant stuff is about practice -- recognizing what information they need. Comparing with Japan, where they always use set phrases (that are really long!) it can often change in US, depending on where you go. I'm sure you'll be fine though!

Thank you.

Enjoy! (Don't eat too much fast food now, though! haha)