Don’t assume that big brands are automatically better than individual agents in Mexico. Since all estate chains operate on a franchise model, each local agency will only be as good as the agent working in it.
A good estate agent in Mexico should have lived in the respective location for various years, know its areas and neighbourhoods, history, infrastructure and development plans, the local culture and the local market prices and conditions. In many cases you will maintain your relationship with your agent even after the completion of the property purchase, i.e. to rent out your property or to sell it later on.
As a buyer you do not have to worry about the agency fees in Mexico as these are paid by the seller of the property (usually 4-8% of the property value). Instead you need to make sure that your agent understands what you are looking for and that he takes a proactive approach towards your property search. The initial response rates are already a good indicator for this: How fast does your agent react to enquiries from your side? Does he also look for properties outside of his direct portfolio?
Of course you can also contact various estate agents at the same time. However you will be probably be better off with one proactive and well-connected agent than with multiple estate agents that only send you their standard offers without further explanations.