Objection 1. It would seem that the guardianship of men does not belong only to the lowest order of the angels. For Chrysostom says that the text (Matthew 18:10), "Their angels in heaven," etc. is to be understood not of any angels but of the highest. Therefore the superior angels guard men.
Objection 2. Further, the Apostle says that angels "are sent to minister for them who shall receive the inheritance of salvation" (Hebrews 1:14); and thus it seems that the mission of the angels is directed to the guardianship of men. But five orders are sent in external ministry (112, 4). Therefore all the angels of the five orders are deputed to the guardianship of men.
Objection 3. Further, for the guardianship of men it seems especially necessary to coerce the demons, which belongs most of all to the Powers, according to Gregory (Hom. xxxiv in Evang.); and to work miracles, which belongs to the Virtues. Therefore these orders are also deputed to the work of guardianship, and not only the lowest order.
On the contrary, In the Psalm (90) the guardianship of men is attributed to the angels; who belong to the lowest order, according to Dionysius (Coel. Hier. v, ix).
I answer that, As above stated (2), man is guarded in two ways; in one way by particular guardianship, according as to each man an angel is appointed to guard him; and such guardianship belongs to the lowest order of the angels, whose place it is, according to Gregory, to announce the "lesser things"; for it seems to be the least of the angelic offices to procure what concerns the salvation of only one man. The other kind of guardianship is universal, multiplied according to the different orders. For the more universal an agent is, the higher it is. Thus the guardianship of the human race belongs to the order of "Principalities," or perhaps to the "Archangels," whom we call the angel princes. Hence, Michael, whom we call an archangel, is also styled "one of the princes" (Daniel 10:13). Moreover all corporeal creatures are guarded by the "Virtues"; and likewise the demons by the "Powers," and the good spirits by the "Principalities," according to Gregory's opinion (Hom. xxxiv in Ev.).
Reply to Objection 1. Chrysostom can be taken to mean the highest in the lowest order of angels; for, as Dionysius says (Coel. Hier. x) in each order there are first, middle, and last. It is, however, probable that the greater angels are deputed to keep those chosen by God for the higher degree of glory.
Reply to Objection 2. Not all the angels who are sent have guardianship of individual men; but some orders have a universal guardianship, greater or less, as above explained.
Reply to Objection 3. Even inferior angels exercise the office of the superior, as they share in their gifts, and they are executors of the superiors' power; and in this way all the angels of the lowest order can coerce the demons, and work miracles.
Source: Summa Theologica (New Advent)