I understand that sifardim do yihud after the meal and ashkenazim do it before
What is the reason/source ashkinazim do it before?
Can an Ashkenazi person do it after?
Nitei Gavriel Nisuin 1 - 37:2 says the reason Yichud is done immediately after the Chupa is to minimize the Hefsek between the Chupa and Yichud. He mentions that the Sefardim do Yichud after the Seuda and in note 8 that the German Minhag is also to do it after the Seuda since they consider the spreading of the Talis the main part of the Chupa.