How to make page template with admin able to select NextGen Galleries to be part of the layout?

You can add galleries directly to a template by calling do_shortcode() to run the gallery shortcode.

<?php echo do_shortcode('[nggallery id="1" template="example"]'); ?>

Edit: sorry, writing decaffeinated so short attention span 🙂

Add some custom fields to your product pages to let your client pick which gallery goes on that product. If you like, just use the Advanced Custom Fields plugin and its friend the NextGEN Gallery Field add-on, which will let you add drop-down lists of gallery names to your edit pages.

Then in the template, get the value of each custom field and load the gallery using the shortcode as above, like this (replace ‘gallery1’ with your custom field names):

<?php
$gallery = get_post_meta(get_the_ID(), 'gallery1', true);
if ($gallery) {
    echo do_shortcode("[nggallery id='$gallery' template="example"]");
}
?>