Hi again! Check out that Auto-Res website, click ENGLISH as your language of choice. From the menu, click INFORMATION towards the top. Then, at left, click OUR COACHES, then choose EXPRESS BUS. Wow, those Express Buses are LUXURIOUS! Leather seats?!
Looks VERY comfortable. And yes, all these wonderful Spanish (non-city-public) buses are have LOTS of cargo space in the belly of the bus.
Now, what's the BEST way for a non-Spanish-speaking person to get to the train/bus station? Hmmm.. The best
way.... I suppose it could be to simply tell the taxi driver the name of the train/bus station and let him do the work but I'm not so sure ONE taxi would be able to carry your luggage of one-suitcase-per-person - depends on the size of the suitcases, I guess.
With the luggage you'll have I wouldn't really recommend taking the metro - it'd be difficult to negotiate the metro changes at the different stops.
You could hire AeroCITY
to take you there directly but they mainly only take airport arrivals to hotels however they may be able to give you direct transportation at a different price.
Regarding the 2nd Class Only train - I really don't know. Maybe someone else is familiar with this particular train to Salamanca. My guess is there's only one type of service to/from Salamanca. And yes, I'm certain there's space for your luggage.
Best of luck and enjoy your time in Spain!