"I want some tea."
There are plenty of situations where one would not specify which kind of tea they want. In that case, I would think お茶 would be the preferred term, since by definition, it just means "tea" (i.e. it is the honorific お followed by the kanji for tea, 茶). こう茶 specifically means "black tea" if I'm not mistaken (i.e. a kanji for red, 紅, followed by the kanji for tea, 茶).
少し isn't exactly the same thing as "some". I think Duolingo is using "some" as more of a plural indeterminate article than a quantifier. If they want "少し", they'll probably say something like "I want a little bit of tea". Obviously, it doesn't map well to Japanese; 少し probably should be accepted anyway.
It should probably accept answers without です, but it makes sense to include it in the official answer, since the entire course thus far has been in formal form. Dropping it would indicate the informal conjugation of 欲しい, which would be inconsistent with all the other です and ます translations.