Say both “SamiAllah…” and “Rabbana…” in salah alone? Repeat salah if forgotten?

While praying salah behind imam, after ruku, imam says “SamiAllahu Liman Hamidah” and muqtadi says “Rabbana Lakal Hamd”. While praying salah alone, to say both (SamiAllah…. and Rabbana…….?). But it is to say so both while only in fardh salah or also while praying sunnah and nafl or to say only SamiAllah….. while sunnah and nafl salah? While praying salah alone, if forgot to say Rabbana…. after SamiAllah…. in 1 or in all rakahs, then will require to repeat salah or not? Please guide me.