We'll be using Auto-Res buses to travel between Madrid and Salamanca next month. Will we be able to carry our shoulder bags or back packs into the seating compartment (instead of storing them in the luggage compartment)? And, do the tickets come with assigned seats? Thanks.

It depends on the size of your backpack. If it fits into the compartment above the seat you won't have any problems to store it there.

As for the tickets, yes they are assigned with a number for a certain route, day and time.