lower-ranking vs petty

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

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

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

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

lower-ranking (noun)	petty (noun)
	larceny of property having a value less than some amount (the amount varies by locale)

lower-ranking (adjective)	petty (adjective)
inferior in rank or status	inferior in rank or status
	contemptibly narrow in outlook
	(informal) small and of little importance