1- Regarding the number of Rak’ahs in Taraweeh.
Shaykh al-Islam Ibn Taymiyyah said: If a person prays Tarawih according to the madhhabs of Abu Hanifah, al-Shafi’i and Ahmad, with twenty rak’ahs, or according to the madhhab of Malik, with thirty-six rak’ahs , or with thirteen or eleven rak’ahs, he has done well, as Imam Ahmad said, because there is nothing to specify the number. So the greater or lesser number of rak’ahs depends on how long or short the qiyam (standing in the prayer) is. (Al-Ikhtiyarat, p. 64)
2- Praying behind a Shafii Imam who does Qunoot in Fajr.
Imam Ibn Taymiyyah said: “Similarly, if the worshipper follows an imam who does qunoot in Fajr or Witr, he should pray qunoot with him, whether that is before bowing or after, and if he does not do qunoot, he should not do qunoot with him. If the imam thinks that it is mustahabb to do a certain thing in prayer but the one who is praying behind him does not think that is the case and he (the imam) decides not to do it for the sake of harmony and unity, that is better.” [Al-Fatawa al-Kubra, 2/476]
3- Praying Witr like Maghrib because the majority in the congregation want the Imam to pray so:
Ibn Taymiyyah said: If the imam thinks that it (i.e Witr) should be done separately, and the congregation thinks that Witr should be done like Maghrib (like Hanafis), and he agrees to do that so as to create harmony, that is better, as the Prophet (peace and blessings of Allah be upon him) said to ‘Aa’ishah: “Were it not that your people have only recently left Jaahiliyyah behind, I would have razed the Ka’bah to the ground and rebuilt it with two doors, one through which the people might enter and one through which they might exit.” But he decided not to do that which was better, lest it put the people off. End quote. (Al-Fatawa al-Kubra, 2/476).
(Compiled from Islamqa.info.)