May one use mezuzot with different scripts in the same house? For example can the muzuzah in one room be ashkenaz (beit yosef) and in another be sefardi (vellish)?
Halakhicly speaking it is fine. Since in the time of Hhazal there were no such cultural/ethnic divisions, no ruling (that I know of) was made on this matter.
Nor is this to be found in the earliest halakhic codifications (ie. Sepher Ha-Halakhoth of the RIF and Mishneh Torah of the RaMBaM)
It is also interesting as a side question as to what other ethnic groups like Mizrahim and Chabashim would do if they could not get mezuzoth according to their mesora and were faced with a "Ashkenazi or Sephardi" situation I assume they would take the irritating Israeli approach of "if their not from Ashkenaz their Sephardi" OR should they mix?