subaltern vs lower-ranking

subaltern is a noun but lower-ranking is not a noun.

Word	Noun	Adjective	Verb	Adverb
subaltern	Yes	Yes	No	No
lower-ranking	No	Yes	No	No

As adjectives, subaltern and lower-ranking are synonyms defined as:

Other synonyms of subaltern include junior-grade, lowly, petty, secondary.

subaltern (noun)	lower-ranking (noun)
a British commissioned army officer below the rank of captain

subaltern (adjective)	lower-ranking (adjective)
inferior in rank or status	inferior in rank or status