Entry for "Angels" in Tolkien Encyclopedia (written by Jared Lobdell):

In The Silmarillion, in Tolkien’s
description of the creation of the Ainur—the Valar
and Maiar before the beginning of time—we find
that the ‘‘Wisest of the Maiar was Olo´ rin,’’ a name
Gandalf said (S, 30, 279) was his ‘‘in his youth, in the
West that is forgotten.’’ In the chapter ‘‘The Istari’’ in
Unfinished Tales is a rough version of a narrative
describing the council of the Valar in which they
discuss sending emissaries (messengers ¼ [Gk] aggloi
¼ angels) to Middle-earth to help in the struggle
against Sauron: ‘‘Who would go? For they must be
mighty, peers of Sauron, but must forego might, and
clothe themselves in flesh so as to treat on equality
and win the trust of Elves and Men’’ (UT, 393). A
peer being someone of equal stature with another,
and Sauron being himself a Maia (‘‘in his beginning
he was of the Maiar of Aule¨’’ in The Silmarillion), the
Valar clearly intended to send other Maiar as their
emissaries (Istari). Tolkien wrote elsewhere that ‘‘we
must assume that they [the Istari] were all Maiar.’’

Entry for "Maiar" in Tolkien Encyclopedia (written by Jonathan Evans):

The Istari, apparently a special suborder of the
Maiar, are an unspecified number including those
named Olo´ rin, Curunı´r, Aiwendil, Alatar, and Pallando.
These five become the Wizards of Third-Age
Middle-earth. In Valinor, Olo´rin—the wisest among
the Maiar—lived in Lo´rien and went often to house of
Nienna, the Vala of grief and mourning, where he
learned pity and patience. In Middle-earth, Olo´ rin
comes to be known as Gandalf. Likewise, Curunı´r
becomes Saruman, whose interest in forging and artifice,
as well as his eventual affinities with Sauron,
suggests an original association with Aule¨. Aiwendil,
or ‘‘bird-friend,’’ becomes Radagast and seems to
have been a servant of Yavanna. Alatar and Pallando
go to the east but otherwise are not mentioned in the
canon of writing on Middle-earth. Sauron, servant
of Melkor, was originally associated with Aule¨ the
smith, a relationship echoed vestigially in his friendship
with the Elven-smiths of Eregion and his skill as
the artificer of the Rings of Power.

Very interesting Jlong. I realize that i wasn't thinking about something very important about the Istari when i posted, "For they must be
mighty, peers of Sauron, but must forego might, and
clothe themselves in flesh so as to treat on equality
and win the trust of Elves and Men’’ (UT, 393). This would then explain perhaps why Sauron seems so much more mighty than even Sauruman, the mightiest of the Istari. He (Sauron) did not forego any of his might, he came to Middle Earth as Melian did in full strength.
It is still interesting to me that JRR talks about the "Lords of the Valar" and states that we must assume that the Istari were all Maia. Definately leaves room for some sort of 'middle class' of Ainur, not Lords of the Valar and not servants of the Valar.