What is reasonable asymmetry?
As you know, it really depends on who you are asking. And everyone will say asymmetry is normal but if its really noticeable to everyone, then its no longer is 'normal' in my opinion when resulting from a procedure. Your surgeon should be able to admit if not right and work with you to make it better since his reputation hinges on the quality of his work. But nipples off a cm, areolas not exactly the same, or one breast larger is entirely normal.
Breast symmetry after reduction
At six months you are not really going to see any change in size or symmetry. The assessment of symmetry is difficult, especially without seeing you in person. The most important measure of symmetry is overall volume. Does one side feel tighter in the bra? Are the nipples at the same height? Breast width is more difficult. Be sure to revisit these things with your surgeon. If things are not helpful with him consider a second opinion with a board certified plastic surgeon with experience in breast revision surgery.