Yes. Provided that you meet the criteria for hiring, you can start using the extra slots to hire additional retainers immediately after subscribing to the Retainer Service.

*Please note, players must first complete the FINAL FANTASY XIV: A Realm Reborn main story quest "The Scions of the Seventh Dawn" before the option to hire any retainers becomes available.

For more questions regarding the Retainer Service, please check the Retainer Service FAQ.