In UT it gives a note written in 1972 " We must assume that they were all Maiar, that is persons of the 'angelic' order, though not necessarily of the same rank....Saruman is said to be chief..Gandalf was evidently the next. Radagast is presented as a person of much less power and wisdom..."
The letter was dated 1956? In that case, it seems to me that it is an inconstancy in use of vocabulary rather than concept. In the note of '72 he uses order to refer to the "Maia" collectively and rank to distinguish between power levels within that order. In the letter he uses kind to refer to the Maiar and order for the divisions... given the 16 year gap.... I think we can let him off... it is clearly the same idea
http://www.sarahsarchangels.com/archangels/9orders.html