Hostels from Toulouse southward

I think this is an incomplete list, but it's a place to start your research.

http://www.chemindecompostelle.com/Selection/CarteFrance.html

I too am doing some research on this route, which is known as the Arles Route, or Chemin de Arles. The web site chemindecompostelle takes you to the Miam Mian Dodo organisation, and it's web site for this part of the route you are interested in is still to be completed.
I suggest you try the Confraternity of St James site, which is
http://www.csj.org.uk.
From their home page click on Routes, then Arles Route. there is a brief overview, which contains information on web sites and Guide Books. One guide book is in French, and the other is a publication in English by the CSJ 'Toulouse to Puente la Reina', obtainable by ordering on line from the CSJ at a cost of 5 pounds.
Hope this helps. David

If by "south" you mean toward Oloron/Ste. Marie and Puenta La Reina, I walked this route in the fall of 2008, and used the Miam Miam Dodo guide that you can see in the Kitsamble recommendation. It was a very thorough guide book in French. There is a new 2010-2011 edition available through http://www.amazon.fr.