Gallery shortcode questions

Hello.

I am using the gallery shortcode and edited the includes/media.php to generate the code I want.

Two questions:
1 – can I display just some of the images I uploaded? For example, I upload some images to a page for custom fields to use and I don’t want them to display in the gallery.

2 – the gallery links to the file(image), not the attachment, and potentially the images could be very big. Can I get the gallery to link to the ‘large’ or ‘mediuim’ version of the uploaded gallery image?

here is what I am using at the moment in my template:

<?php echo do_shortcode('[gallery size="thumbnail" itemtag="ul" icontag="li" captiontag="p" link="file"]'); ?>

Thanks