lower-ranking vs subaltern

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

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

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

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

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

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