What should one do if he is a guest by someone’s house, or comes to someone’s simcha, and the host makes Kiddush on a small cup of schnapps, as is the Chassidic custom? May one rely on this Kiddush and eat in order to avoid embarrassing the person? One could ask for wine, but perhaps wine is not available or perhaps the person would be insulted. (The mesorah of the guest is that Kiddush is said on a reviis of wine.)
Being that this person doesn’t have a mesora to be lenient and use a cup less than a revi’is he should not be meikel, unless there is absolutely no other option. There are indeed poskim that say that he s not yotza at all even if the host made kiddush this way. If the person can tell the host that his minhag is to only make kiddush on a reviis of wine or grape juice, he should do that, and not be meikel, because most poskim hold that it is not permitted to make kiddush on less than a reviis. As a side point, if it would be against your minhag would you eat gebrokt, or kitnios on Pesach because the host does? No. You would explain it to him that your minhag is not this way. If the host has wine he will gladly give you some to make your own kiddush. You also have the option to make kiddush and drink a maleh lugmav yourself, (just control yourself during the meal).
Ketzos Hashulchan 95 ftnt. 5, Kovetz halachos (Shabbos) 12 last paragraph of ftnt. 14, and other poskim.