B"H, 16 Iyar, 5711.
Brooklyn.
To the student Shmuel Menachem Mendel Liberov
Shalom u'Vrachah!
...Regarding what you ask about studying Chumash with Rashi: in general, it is customary to learn the daily portion of Chumash during the day, and if one does not manage to complete it during the day, it can be made up on the following night... I have not heard that people are particular about the order of Chumash, Tehillim, and Tanya.
With a blessing for proper success in all matters.
Summary
The Rebbe clarifies that while there is a custom to study daily portions of Chumash with Rashi during the day (and make up at night if needed), there is no strict requirement regarding the order in which Chumash, Tehillim, and Tanya are studied.