Mexican labor law can be quite confusing, but it’s very important to understand your rights as well as the rights of your workers. A good place to start is by reading articles such as this one at ICLG.
When in doubt it’s always best to contact a lawyer. Hiring, firing, promoting, deomting, raises, IMSS, etc. etc. can get complicated and land you into trouble if you don’t do things correctlty. Always be diligent!