If the arrest itself was legitimate (obviously that assumes a lot), then they had the power to arrest you. The only issue then was whether they really needed to have you cut your fingernails. It's hard to say since I don't know what they'd say about why they couldn't do the fingerprints with them long, but if you feel they acted improperly, file a complaint with the Civilian Complaint Review Board: http://www.nyc.gov/html/ccrb/html/home/home.shtml