Essentially, tales are stories set the universe that follow formats not adhering to SCP articles or GoI formats.
Beyond that, the definition is quite broad. Tales are, honestly, the most liberally formattable articles on the site. You can structure them more akin to a conventional literacy style, or do something different, such as diaries, logs, etc. The absolute best way to understand is simply to read tales yourself.
Also, I'm not sure if it's possible for it to be narrated by someone outside of the foundation
I don't see why not? Many tales do this already.
Asking because there are events happening outside of the foundation that still revolves around my SCP and is vital to how it came to be and what could happen in the future but It doesn't sound like something you would read in an SCP article.
First off, if the information you're planning to sideline into a tale is "vital", then my suggestion would be to include it in the original article. A reader is not obligated to read another piece of work to understand the current, and leaving a chunk of story out is unnecessary, at best.
Tales based on SCP articles should serve as enhancements, an extension, if you will. Perhaps stories playing around with the concept, or answering questions that weren't dealt with in a satisfying way.
Secondly, I'm not sure what you're saying, that this "vital" information is somehow not appropriate for an SCP article, but having the tale be journal based? Journals are arguably the easiest to integrate into an article. Beyond that, there are still numerous ways to tell a superficial story through the addenda. The best way to learn about narrative integration is by reading modern works (see: User-Curated Lists) or going through Guides for Writing on the Guide Hub
