The "-sha" pattern is relatively consistent: Pavel-Pasha, Mikhail-Misha, Natalya-Natasha, Nikolay-Nikolasha, Alexey-Alyosha, Mariya-Masha, Ilya-Ilyusha.
So, Aleksandr-Aleksasha. The dropping of "Alek" is the only inconsistent part, on par with Agrippina-Agrusha-Grusha.
Interesting, never heard 'Nikolasha' once
> Interesting, never heard 'Nikolasha' once
It's archaic, used in Peter I times. Modern one is Kolya
It is not common. Usually you say Kolya.
Ditto.