Of course, it depends what you mean by "guardian angel." May I say that I do not share your typically female over-romanticised view of either angels or Life In General and therefore probably cannot supply the answer that you wished to your question. However, going on the ideas of Man, it follows thus:

Devils, by deffinition, are "evil," having chosen to fly down into Hell with Satan, and so on. As Heaven/Hell is outside the normal temporal pattern of the Earth they cannot say that they were cast there as Devils later on, or due to actions on Earth, for they in truth have been there since the Heavenly Schism. Therefore, and using the typical deffinition of a "guardian angel" as a protector and a guide; even, perhaps, a servant; Devils - or beings of Hell - cannot possibly be considered such. Furthermore, to take the common view of the Heavenly Schism and of Devils/Satan, the very reason that they were/are cast into Hell was because they refused to be subservient to humans (i.e. be guardian angels).

To follow up more on your apparant interest in Devils and Demons, Jordyn, I would heartily and sincerely recommend the book "The Screwtape Letters," by that most emminent and excellent of Christian Writers, C. S. Lewis. The book is a compilation of "letters" written by a senior Devil - Screwtape - to his young nephew Wormwood, who is just beginning his career as a Demonic tempter. The book discusses Devils' methods for tempting men away from Christianity and into the pits of Hell, picking up "What Christianity Really Is" (Lewis' view) on the way.