This is a really interesting post that made me think about my own travel style. I like at least one solo trip a year for the complete freedom from compromise that it gives me.
I usually try and avoid any more than perfunctory contact with other travellers and stay in small business hotels hoping to avoid other English speaking guests as much as possible.
There are still enough interactions with other people to satisfy that need for human contact but without having to do something or go somewhere you don’t really want to.
The only times I could do with company are on weekend evenings or in holiday resorts where everyone else seems to be in a group.
I went solo to a trendy Wok restaurant in Bogota on a Saturday night and the waitress actually laughed out loud when she realised I was alone.
For those reasons I think your trip to Spain sounds great, to go your own way during the day and an arrangement to meet in the evening must be the best of both worlds.
However the idea of sharing a room or a dormitory with someone unless it was someone more than just a fellow traveller would never suit me. That mental space when you shut the door behind you is one of the highlights of solo travel.
