I think it really depends on who you get at the counter when it comes to mixing and matching sides. There have been times I've asked (for example I don't like the pasta salad but really enjoy the Italian sub, so I've asked to get fries instead), and I've been told yes, and times I've been told no. I don't think there is a hard and fast rule, at least not one enforced consistently. I do also have the suspicion that if you got your sandwich, then walked over and got a side of fries, and then walked over to the check out, that you'd likely meet with no resistance.
Overall I've been really happy with the dining plan, we've eaten something like 25 meals in the parks, so I've gotten my money's worth, and I've found the food to be consistently decent. I do hope they continue to have the funnel cakes on the season dining menu, because we'd often come to the park, have an early lunch, and then use our next entitlement for a funnel cake before we left for the day. With two small kids, we can't do all day, and pretty much never stay until dinner time, but those funnel cakes were a nice afternoon snack!
