As I’ve almost written in the title:
Can charr from different High Legions, join together in the same Warband? Or must Warband members be of the same High Legion?
Thank you in advance 
Warband members from different High Legions?
I’m hardly an expert on Charr lore, but… I know you end up recruiting at least one “other legion” Charr into your warband during your personal story no matter which legion you play through. At the very least, I distinctly remember recruiting an Ash Legion soldier into my Blood Legion characters’ warband. So it definitely happens.
Not sure what happens once you get recruited into a warband of a different legion, though. Whether you stay aligned to your original legion or change to the legion of your new warband. I would imagine once you join a warband you switch legions to match the warband’s as well, though. So if a Blood warband recruits and Ash soldier, said soldier becomes Blood Legion.