Calliope, Kanaya and Jade lived in remote and isolated locations with only 1, 2 or no other guardians living with them, do you think this was to reflect how isolation is tied to space?

It’s hard to say at which point Hussie decided Space = Isolation. It could very well be a theme that was introduced with Kanaya, and Jade just happened to retroactively make sense, or maybe it was planned from the beginning. Either way, it does fit in well with their nature.