I'm so glad I found these guys, 10/10 experience!
We did the free walking tour with Farid (recommend doing this early in your trip to help set the scene for the city and the culture). He was so knowledgeable on the history of the city, shared his recommendations for food and wine and also taught us about the animal guides of the Zapotec culture which was really interesting, of which you see many of these animal figurines on sale in the shops around town. After learning about them and finding out which was my animal guide, I knew which animal figurine to buy which made it a bit more meaningful of a souvenir for me.
The next day we took the full Hierve el agua tour with Miguel and another guide-in-training (so sorry but I forgot his name - Carlos?) and both were excellent and really made the long day worthwhile and entertaining. I'm astounded at the fountain of knowledge Miguel seemed to have! I felt the tour was overall well paced and enough time was spent at each stop.
Can't recommend these guys enough!