You can get a specific credential for the El Salvador at the Santa Maria Albergue in leon, it cost 2 e and gives info on the route, ie albergues, distances, elevation profile, facilities along the way!
The way is very well marked, no other guide is necessary! I walked this route beginning of June, took 5 days, but possible to make it 6/7 days also. Its a very nice camino and you can get a certificate in Oviedo when you get there!
Buen Camino
Aidan