Sajdah and Jalsah

Reciting ربنا و لك الحمد حمدا كثيرا طيبا مباركا فيه in fardh Salaah

Q: Can a person read the following Dua in his Fard Salaah:  حمدا كثيرا طيبا مباركا فيه after reading: ربنا و لك الحمد, in ruku', similarly can he read in jalsah: اللهم اغفرلي و ارحمني و عافني و اجبرني و ارزقني and if he is a muqtadi can he read these two dua's?

Method of sajdah

Q: When I make sajdah, I am not satisfied and I push my head too hard. I want to know the correct way of making sajda. How much pressure is requried? Where exactly should the floor touch the nose and forehead? Anywhere or some precise position is required?

Reciting Sami Allahu liman Hamidah instead of Subhana Rabiyal A'laa

Q: When performing Salaah behind the Imaam, in the sujud after saying a few times "subhana rabbiyal alaa" somehow I was about to say "sami Allahu liman hamidah" but as soon as i said "sami Al" I realisized and cutt off and continued saying "subhana rabbiyal alaa". Now was my prayed invalidated since I cutt of at the half? Must I repeat it?