maho, we are not totally comfortable with counting uncountable, so all such expressions are considered 'somewhat incorrect'. Though they are used of course. Why I say this is... the likelyhood to say it this way depends on: The situation, the liquid in question, the way this liquid is normally served, you habits and how often do you order it. I personally would say 'один Guinness', but 'Чашку кофе'. Well, normally the have two kinds of glasses for beer (0.3 and 0.5), so I would specify here too:/ один кофе OR одно кофе. Until recently people insisted that the true gender of кофе is masculine, while its usage in neuter, though common, was considered 'colloquial' and 'uneducated'. Now both genders are officially accepted as a part of the norm. Still many people avoid and crticize the neuter form (which is more common). So masculine is somewhat more 'learned':) OR одну чашку кофе. - Одну водку, пожалуйста (it is accusative case: an omitted part is 'give me', thus accusative) - Одну рюмку/стопку/бутылку водки пожалуйста etc. (стакан, графин...) - ХХХ грамм водки, пожалуйста. - 100 years ago it also could be один штоф водки)

Yes, you're right. But everywhere Accusative case should be used. Because actually you mean "Give me a beer, please" (Give + what? --> Accusative case). Одна водка --> Дайте одну водку. BUT in real life when asking for something, we just skip the phrase "give me" and say just the noun we're asking for! But accusative case should still be there as it's a request for that noun! "(Дайте мне) одну водку, пожалуйста". // "Одну водку, пожалуйста".

yes. you have to adapt одно to substansive, but also to падеж(case I think). одно пиво- neuter, accusative один кофе- masculine, accusative одну водку- feminine, accusative одна водка is neutrum. but even if you say одна водка it could not be mistake. In speech its fine. You even could say just водку пожалуйста. Одну is not necessery