The different names with which angels are called reflect their [spiritual] levels. Thus, they are called:
1) The holy chayyot, who are above all the others;
2) the ofanim;
3) the er'elim;
4) the chashmalim;
5) the serafim;
6) the mal'achim;
7) the elohim;
8) the sons of the elohim;
9) the keruvim;
10) the ishim
These ten names which are used to refer to the angels reflect their ten [different spiritual] levels. The level above which there is no higher level except that of God, blessed be He, is that of the form called chayyot. Therefore, the prophets state that they are below God's throne of glory.
The tenth [and lowest] level is that of the form ishim. They are the angels who communicate with the prophets and are perceived in prophetic visions. Therefore they are called ishim, because their level is close to the level of human knowledge.
These ten names which are used to refer to the angels reflect their ten [different spiritual] levels - We do not find such a listing of the different levels of the angels in the Talmud or Midrashim. The list mentioned by the Rambam is found in the Zohar (Vol. II, 43a), although in a slightly different order.
The Sages of the Kabbalah explain that all existence is an expression of the ten sefirot (emanations) of Godliness. Though these sefirot relfect different qualities of God, and thus transcend entirely the levels of the angels, we do see a certain commonaility between these mystic teachings and the Rambam's conception of the spiritual realms. (See also the Guide for the Perplexed [Vol. II, Chapter 4], which mentions ten levels of spiritual existence).My personal opinion is that the list of ten races of angels is interesting, I don't think that the list is meant to be in any way exhaustive in terms of non-human entities that we can interact with. Also, the letters of of the Aleph Bet are angels too. Personal experience is the best teacher in my opinion. As Jack put it:
Go out, have an adventure... See what you live beside. Magick is about living, and seeing differently. At least sometimes, anyway.