Order of reading surahs in salaah

Assalam o alaikum,Respected Mufti Sahab, may my mail find you in your best of health and Imaan. I have a question related to Namaaz. Suppose I read Surah Kauther?in the first Rakat of Fardh Namaaz of Zohr whether I am praying my Namaz alone or whether I am leading in a prayer, in the second Rakat can I read Surah Kafiroon or not? If I read it will my Namaz be valid. Some say that after a small Surah you should not read a big Surah, is it true or can I go ahead with it. Can one read it in any Namaz like Fardh, Sunnat or Nafil.
I have read a small Surah or any three Aayats from the Quran then after that can I read a big Surah in the second Rakat or this will nullify my Namaz. What if I do read it; do I have to repeat the Salaah?