Regarding Ameen after Surah al-Hamd, there is consensus of the imams of ahl al-Bayt that this is prohibited. This is for a number of reasons: 1st, Allah prohibits speech in the prayer--refer to the tafseer of Qur'an 2:238. 2nd, Allah commands that we be silent during the recitation of the Qur'an--see Qur'an 7:204. 3rd, the Prophet (sawas) said in an authenticated hadith: ((No speech is suitable for the prayer. The prayer is only recitation of the Qur'an, tasbeeh, tahmeed, tahleel)) and in some narrations ((...takbeer...)). 4th, those narrations that say that Ameen was recited are all weak. 5th, there is a hadith where the Prophet (sawas) said: ((When the imam says "...wa ladhdhaaleen" listen silently)).