Would it be possible through the Content Manager/Content/Site Map to link a CMS page with a WYSIWYG page and vice-versa? You can have as many WYSIWYG pages linked with one another, but no assocaition with a CMS page. At the moment you can only use a CMS page for it's intended purpose (e.g. the Gallery only accommodates photo - yet if you wanted to link it to a WYSIWYG page with embedded YouTube Vids you can't). And there be other types of associations where this may be desirable.
On a specific point, is there any other way that I can embed a YouTube link in the CMS gallery page? Or create a hyperlink from an exixting photo to an YouTube vid?
To avoid confusion of terminology (hopefully!) I'm going to refer to CMS pages as the pages you create in the Site Map using the Content Management tab, and Application Pages to be those pages that are in an application, such as Photo Albums and Forums.
You're right that you can't add includelets to Application Pages, and nor do Application Pages appear in things you create with the CMS, such as Filtered Item Lists. This is a bit annoying and is to do with the way the toolkit works, and as such I don't think it will ever fundamentally change.
The problem is that in the database a CMS item like a Page or Asset is completely different to a Photo in the gallery, or a Forum message, or a Blog post, etc. Although they share attributes (so that things like the permissions and search indexing work correctly) they are created and managed by different applications and the other applications don't know much about how to handle them.
There are a few special cases - for example the Photo Album or Surveys includelet has limited knowledge of the page types from other applications so can show them on a page.
So it is possible to have some limited intermixing of application pages in the CMS, but it's probably impossible to ever have CMS pages served up through the applications.
So I think the best thing to ask here really is - what do you want to achieve? Maybe there's a way to achieve what you want with the system as it is, or maybe with a few minor alterations, such as new Includelets, we can make it do what you want?
On your specific question - unfortunately the photo album doesn't support videos directly, but maybe there's something that we can do here to improve this, if this is what you're looking for? Either by allowing videos to be uploaded directly into the gallery, or by allowing you to specify a YouTube video id as a "photo" to be displayed!? Probably the former makes most sense, as long as the videos aren't too huge!