I read there are three versions of Mezuzah writing stumah and ptuchah between parshiyot: Rambam, Rosh, TaZ. Do Ashkenazim accept an opinion of TaZ (David ha-Levi Segal)?
Yes, the Taz is fairly standard for Ashkenazi Mezuzos today.
There is a dispute between the Rambam and the Rosh concerning the definition of Setuma and Pesucha Parshios.
According to the Rambam, whenever a Parsha starts in the middle of the line it is considered a Setuma. The Rosh maintains that this is the layout of a Parsha Petucha. Also, according to the Rambam whenever a Parsha starts at the the beginning of the line it is Petucha. The Rosh says that this is a Parsha Setuma.
Sephardim write Mezuzos, where the Parshios must be Setumos, in accordance with the Rambam. Ashkenazim are concerned for the Rosh, and the Taz created a kind of compromise that he maintained will fulfill both opinions. This is followed by most Ashkenazim (though some follow the Rambam).
For purposes of Mezuzah both versions will be kosher bedieved, whereas for Tefillin the matter takes on greater gravity.