In Igros Moshe Orach Chayim 4:9 (pg. 14), it discusses in brief the importance of having all the mezuzahs and tefillin parshiyos the same ksav. In there it was specifically referring to ksav Beis Yosef not being mixed with Ksav Arizal (where one main difference is the letter tzadik) and that it would be pasul mdin menumar.
I am interested in any marei mekomos that discuss the issue of menumar for mezuzahs. Is this law of having the same mezuzahs a hiddur or chumra or mi-ikur hadin - for within the Igros Moshe it uses the wording posul mdin menumar?
Also would there be a bigger issue when there is Beis Yosef follower using non Beis Yosef ksav in some mezuzahs (or Tefillin) or would it also apply within Sfardic ksav with Arizal?
Of course, all ksavim are kosher and good, but I am asking strictly in regards to menumar.