Do heterodox have a guardian angel as well? (Eastern Orthodox perspective) According to the Eastern Orthodox Church, do non-Eastern Orthodox have guardian angels as well? By guardian angel, I mean a righteous spirit assigned to a person by God in order to guide and protect them throughout their life for the purpose of their sanctification. I know that all Orthodox have them, and I believe they are assigned at Baptism, so it would make sense that non-Orthodox would not have one but I am unable to find a clear answer.  

Bonus points if you can answer regarding confessions that are not in communion with the Eastern Orthodox but whose Baptism is recognized as valid by them, e.g. Roman Catholics, some Presbyterians, etc.
Of course.  God does not have one oekonomia for Orthodox Christians and another for non-Orthodox.   

The Orthodox doctrine of guardian angels is witnessed in the Old Testament as well as in the teachings of Jesus, both of which precede the foundation of the Christian Church.  The Psalms witness:  


  He shall give his angels charge over thee, to guard thee in all thy ways  (Psalm 90:11 LXX; Q. 105, The Longer Catechism of the Orthodox Church, Metropolitan Philaret).  


Christ taught that each one of us has a guardian angel:   


  Take heed that ye not despise not one of these little ones: for I say unto you, that in heaven their angels do always behold the face of My Father, Who is in heaven (Matthew 18:10; Q. 106, The Longer Catechism of the Orthodox Church).  


The latter passage was spoken in regard to unbaptized Jewish children.  

Regarding the applicability of Met. Philaret's catechism, he states that his catechism is addressed to "every Christian", not just Orthodox Christians (Question 1).  Question 106 (and answer reads):  "Has each one of us his guardian angels?  Without doubt.  Of this ye may be assured from the following words of Jesus Christ ..."

Furthermore, the more recent Orthodox Dogmatic Theology of Protopresbyter Michael Pomazansky states (p.123):


  The Orthodox Church believes that every man has his own guardian
  angel, if he has not put him away from himself by an impious life.


It was brought to my attention that a particular Russian Orthodox priest published on his website an explanation:


  In the prayers for the making of a catechumen, we pray “Yoke unto
  his/her life a radiant angel…” From this point onwards, we certainly
  have a guardian angel who protects us in ways we cannot fully
  understand.


which leaves the impression that there is some Orthodox teaching that one only receives a guardian angel if one enters the Orthodox Church.

The statement is correct in explaining that our guardian angel protects us in ways we cannot fully understand upon entering the Church (although technically, catechumens are still "outside" the Church), but the priest would not have meant that one doesn't even receive a guardian angel until that point.  There is a belief, however, that once one is joined to the Church, spiritual things change in many respects.  Another Orthodox theologian, Dumitru Staniloae, writes:


  In the Church, a communion is brought about not only among her visible
  members, but with the angels as well.  The *sobornicity** of the
  Church takes in the angels too.


(* Sobornost is a slavic term meaning something like "spiritual community").

Evagrios the Monk wrote, as Fr. Dumitru quotes, "Know that the holy angels encourage us to pray and stand beside us, rejoicing and praying for us" ("On Prayer", The Philokalia, Vol. 1, p.65); and "If you pray truly, you will gain assurance; angels will come to you as they came to Daniel, and they will illuminate you with knowledge of the inner essences of created things" (Ibid.).