Hi
My next project is to run underground cables and install subpanel in the small building in the back of my property and eventually convert this structure to a guest house. Question: Are there any rules about underground cables in Merida? How deep? And what kind of conduit to use?

To be absolutely safe, if exposed or installed in shallow ditch you can run your wires in the galvanized rigid conduit.

Hi
There are no rules that I know of. But there is common sense and even if no one will inspect your new lines you should run it deep enough so you or your gardener will not cut into it while planting something (If you can’t bury it more than 2 feet, cover it with some concrete),
you should run it in the PVC green pipe (thick wall), try to keep it at least 5 feet away from the swimming pools and other water sources. Also, running one at least # 8 ground wire without the insulation in the same conduit will help. If you can, connect it to a GFIC breaker.