We had a family portrait of 13 people taken off of a pier by the lake in mid-September, the leaves were changing and actually many had fallen off already.
We said we did not want a "stuffy" formal photo as that is not what our family is like. The photographer recommended jeans or khakis--we decided on blue jeans and one person wore khaki pants(my dad --he's not big on having his picture taken so we thought we'd keep him happy

).
The photographer also said for no one to wear bright colors as that draws your eyes away from the group and more towards that one person and it takes away from the photo.
As for the shirts, everyone wore autumn colors--t-shirts, jackets, sweaters, button up shirts--whatever was that persons individual style---that is the one thing we did not want to do with our picture! We did not want anyone becoming/dressing in a way that was not suited to their personality.
The colors varied but were beautiful together (burnt orange-like a rust color not the bright orange, moss green, tans, browns, burgandy-again my dad, just has to be different---autumn, earth tones). We haven't received the proofs yet but the photographer showed us on his screen and the pictures looked amazing. I can hardly wait to get to see the actual photos as this photographer is outstanding!
It's hard to get everyone "on board" when it comes to a "look" but I think maintaining a sense of individuality in your picture for each person is important IMHO as you don't want to look at a picture and say "that looks nothing like Chris--he would never be caught dead in a polo shirt" or something along that line.
Good luck with your pictures and share with us what you decide!