Omitting a Surah in-between two Surahs

Q: I have been told that if, for example one reads surah Nasr in the first rakaat, then in the next rakaat one cannot read surah Ikhlaas. One must recite the surah immediately thereafter or there should be at least a gap of two surahs. Please clarify this rule for me (e.g., does it apply only to short surahs? Does it apply to a 4 rakaat salaah as well?)

Reciting a portion of one surah in the first rakaat and reciting a portion of another Surah in the second rakaat

Q: If a person recites from the middle of one surah in one rakaat and from the middle of another surah in the second rakaat or if a person recites from the end of one surah in one rakaat and from the end of another surah in the second rakaat of a fardh Salaah, will the Salaah be valid?

Two people performing jamaat

Q: If the time for Fajr salaah in the masjid /musallah is up, and there is only the muazzin and imam present, but someone arrives just then but still needs to make wudhu and read sunnats. Should the muazzin stand behind the imam because they know there is another musallee that will join the salaah later, or should he stand on the right of the imam and when the musallee comes to join then move back to the saff?