The following link will take you to the Advanced Customization FAQ in our help pages. If you check out #9 there is information on how to change the way the galleries are arranged.
http://help.smugmug.com/customer/portal/articles/95526
Another great resource for customization questions is our forum, Dgrin.com. Here you'll find thousands of photographers and SmugMug users who have a wealth of knowledge to share. To post a question you do need to register, but its easy to do. This link will get you started:
http://help.smugmug.com/customer/portal/articles/93370-very-cool-forum
I hope this information is useful!
Seth, SmugMug Support Hero