For articles with more than 7 authors, is it compulsory to shorten it or just suggested? I have quite some articles with more than 7 authors. I read here that when an article has more than 7 authors you shorten it in the reference list. So for example: 


  Hampel, H., Y. Shen, D. M. Walsh, P. Aisen, L. M. Shaw, H. Zetterberg,
  J. Q. Trojanowski, andK. Blennow. 2010. Biological markers of amyloid
  β-related mechanisms in alzheimer’s disease. Experimental Neurology,
  223(2):334 – 346. Beta-amyloid and tau protein abnormalities in
  Alzheimer’s disease


becomes:


  Hampel, H., Y. Shen, D. M. Walsh, P. Aisen, L. M. Shaw, H. Zetterberg,
  ... K. Blennow. 2010. Biological markers of amyloid
  β-related mechanisms in alzheimer’s disease. Experimental Neurology,
  223(2):334 – 346. Beta-amyloid and tau protein abnormalities in
  Alzheimer’s disease


if I'm correct. 

Do I have to shorten every reference with 7+ authors this way? Or is this just a suggestion for if the article has an excessive amount of authors (as here where the citation took up half the page)? 

Thanks!
This depends entirely on the style sheet you are using. For example, MLA demands "et al." for three or more authors; APA for six or more. The right thing would be to familiarize yourself with the house style of the publication you are targeting.