Index Pages by tags? Flatiron Theme

Site URL: http://colorist.us

I have an index page with galleries under it. But I want a few pages that mimic the behaviour but only have some of the gallery posts. So on the main site it would have all of them, but then if another navigation page is selected it would be a subset of the posts. For instance separating out blog posts, or photos, or videos. I would assume this could be done with tags, but I made a second index and I don't see any way to do anything but add new pages. I don't want to duplicate the content because then if I need to modify something I'd need to modify it everywhere. Difficult to build all pages twice, not to mention SEO issues.

Is this doable or do I need to ditch squarespace and build a custom site?

Thanks.