The commonsense answer:
If you have already bought the boat, get out and test it, and test pack what you need for 3 to 5 days. The reviews for the eddyline Sitka are good, and it is a nice looking little boat, but I know that some outfitters think that anything under 16′ is just too small. But they like to take lots of stuff. If you are a minimalist you can probably do it.
If you have not yet bought it, a 14′ 6″ boat is probably a bit slower than an 18′ boat, and you may not be able to keep up with the group, but if it is a leisurely paddle this might not be an issue. This of course is dependent upon your technique and fitness level, and that of the group.
As for surf, shorter is better, but if you have no experience landing in the surf, the boat you are in will not make that much difference.
Unfortunately, there is no easy answer.
At the very least it will be a fun day paddling or overnight boat, and if there is nothing else available…