As I am not very fond of
Xander, but
love Alexander, I would definitely use
Alexander as a full name and use the nickname
Xander.
I don't think everyone would start calling him
Alex, unless he told them his name was
Alexander. You can get around this...just use the names
Xander on forms, etc. My friend does this so she doesn't get called
Katherine or
Katie.
Alexander Leo is my favourite of the combos.
Alexander Michael /
Xander Michael is rather boring and it doesn't appear to have a special meaning to you like
Alan or
Leo does (unless there's a special meaning for
Grant).
Alexander Alan is too alliterate, while
Xander Alan is a tad better.