I have not ready anything about Sikhs in the Mexican police either. But you can phone or write to find out.
Customer Service Module:
Ave. Constituyentes No. 947,
Col. Belén de las Flores,
Del. Álvaro Obregón, C.P. 01110, Mexico, D.F.
Phone
(55)1103 6000
Ext. 12603
Monday through Friday from 09:00 hours to 15:00 hours