I booked Severo as well. I did so after speaking with several people that have used him or know him. I got excellent feedback from a gentleman in Houston that used to work at El Pescador. Also got a great recommendation from the host of a well-known saltwater fishing series(Mark Sosin's Saltwater Journal) on cable that used him to shoot an episode a few years back. Severo also ties his own flies. I kind of found that out after he made a shopping list of tying materials for me to bring him. I'm bringing all of my own flies, but it's good to know that my guide is adept at tying, in case I have to make modifications when I get there.
I agree with raskage though. I've contacted quite a few about 8 months back when I started planning and my impression is that there are plenty of good guides that do fly. A lot of guides that specialize in fly are apparently contracting with El Pescador. I called them and they said they would book guides for me even if I wasn't a guest, but informed me that they do reserve their very best guides for their guests. The owner of my local fly shop has used Pescador's guides in the past and told me their guides definitely know their stuff. I'd personally be willing to take his word, since he's the holder of multiple world records on fly. You should try calling Victoria House as well. They can give you contact information for some good fly guides. I hear the people at Ruby's are in the know too, so you might want to try contacting them.