Would just like to know, is the combo of which quilen guardians you get like realm wide and stays the same for the whole lockout or could you realistically try entering with different groups until you get the combo you want?
Not just realm wide: region wide. The entirety of the EU gets the same, America gets the same, etc. They can be different across regions though.
It then stays the same for the entire week: you cannot keep soft resetting the instance to get the combo you want. It will only change when a new raiding week begins.