The answer to If you can use doctor' from social Security in Spain is yes even If you are working with contractors, it doesn't matter becouse for "Hacienda" If you live more than 6 months Per Year in Spain you MUST do the "Declaración de la Renta" so you are paying spanish taxes every year and can use spanish social services.
Rgds
Samca, I think you're mixing "Hacienda" (Spanish Tax Office) with "Seguridad Social" (Spanish Social Security Office). They are two very different things. As you said, if you live more than 6 months in Spain you become automatically a tax resident, than means you have to pay taxes in Spain (to "Hacienda").
You will also have to pay Social Security and that is what gives you the Spanish social security benefits. In the case of a contractor ("autónomo"), those benefits are restricted because you can choose how much you want to pay (kind of). So unemployment benefits, sick leave, pension, etc. will depend on how much you pay every month to the "Seguridad Social".