Granularity of content types Let's say you want to have a set of multimedia galleries, that contain a texts, photos and some youtube films. At the phase of creating content types, I can imagine few approaches. For example - we can create the whole gallery as a one content type, with few optional multivalue fields, or we can create separately content types for photo gallery, separate content type for texts, and all that would be linked by using taxonomy, or we can go even more granular - with content type for ONE photo, ONE text etc, and all that would be linked with each other by taxonomy. How granular content types should be and what are advantages?
As Eugene Fidelin said, we'd really have to have a better feel for the site in order to give you the best advice. It depends - will each gallery have the same fields for each entry? Or does one gallery have videos while another does not? However, I will try to do so anyway.

If there are only a few galleries AND they differ in their fields, I would create a content type for each gallery entry and use them to populate a page with a view. I would do this because otherwise you're going to have to create a single content type with each type of field used by every type of gallery entry, and then for each one you're going to end up having a bunch of empty fields since they are used by the other types of entries.

However, assuming each gallery entry across all galleries has the same fields for the most part, I would just create one content type for gallery entries and then use taxonomy terms and then just use radio buttons to specify what type each entry is (so you might have Animals, People, Places, etc.) and then have views for each type that just filters by the taxonomy terms.