You can get an iced latte with any flavor syrup, and it'll already be sweetened if you order this way...
Here are the flavors:
Classic (which is just liquified sugar)
Peppermint (some stores have this year-round, some only at Xmas)
When you order, ask for an Iced (Tall, Grande, or Venti) (flavor syrup here) Latte.
And no, the Americano isn't already sweetened- it's just espresso shots (how many depends on the size) and water.
Iced coffee isn't already sweetened either, but you can ask for any of the syrups above to be put in your Americano or coffee (iced or hot).
I work at Starbucks