If I mash my snippets now, will I be able to do it again later?

You can create a mashed video at any moment and as often as you want—mashing is unlimited. 

After creating your video, you can continue adding new snippets to the same project and create new mash whenever you like—including the snippets you've already mashed. For example, you can create monthly mashes and keep adding snippets to each passing month, and at the end of the year, create a video that includes all of your year's snippets.