Adding Photos To Collections

I'm trying to set up a collection for my school photos customers. I would like them to be able to choose one 8x10" print (class photo only) and 3 digital downloads (of the individual portraits or sibling photos).

So far I've managed to set up a collection which includes:

1x class or group print

3 x digital downloads (3 different poses)

I'm struggling with the following:

1) Is there a way I can stop a customer from selecting an individual portrait or sibling photo for the class print option? Taking into account that I want to be able to offer a la carte for individual portraits and sibling photos (as different sized prints or digital downloads or both).

2) Is there a way I can stop the customer from selecting 3 of the same photo for the digital download? For example if there are 3 different poses (JohnSmith1.jpg, JohnSmith2.jpg and JohnSmith3.jpg), what is stopping the customer adding JohnSmith1.jpg three times into the collection? Since they are digital downloads, I only want them to add one of each pose.

Thanks in advance.

Claire