When I walked into Salamanca, it was graduation, first communion, weddings, all at once. The city was in full bloom. The albergue had a couple of beds, but we were a group of 5 so three of us went hunting. We wound up in a youth hostel that is not on your list, but it was quite nice. Name is REvolutum Hostal
http://www.hostelworld.com/hosteldetail ... MgodmEUAdQ
It is in a great location, around the corner from the Plaza Mayor.
As far as rooms in a hotel go, May 1 is a holiday and may be tough to find a place. August should be much easier. I decided to spend an extra night in Salamanca and on a whim checked out a 4 **** hotel next door to the hostel. Hotel Rua, very nice, great breakfast, and it was 50 E for a single. This may have been a cheap last minute rate, but given the economy in Spain, cheap last minute rates may be easy to find.