Hi,
I have a blog, and I just use the default taxonomy (Tags and Category)
Now I wan’t to add a “guides”/“Wiki” page.
I see a few ways to do this.
create a /content/guides folder and create 2 new taxonomies called guidetags and guidecategories
use the existing category taxonomy, and call the category “guides”
or create a new taxonomy called “page” and have 2 available values in there, Blogs and Guides
Is there anything obvious I missed here.?
Decide the output of the various pages that make up your guides section, and work back from there. Maybe you don’t want a taxonomy, or can use an existing one in an interesting way.
I’d ask the question, “what is a guide?”
For me, it is probably a single page, in its own section, sharing the taxonomies with the rest of the site, and listing descriptions of the guides on listing pages (think paginated archives). Now we know how I’d add a guides section. 
I do something similar on my blog. In addition to my general blog entries, I have a number of “knowledgebase” sections.
Each is a folder:
content
kb
hugo
....
and each folder has an _index.md file of course.
I chose to keep with just the standard categories and tags taxonomies - for simplicity to begin with but then I decided I prefer it that way and in fact my front page indexes all page updates.
The index pages for each kb area just lists the latest posts for that area.