Welcome @Jwjimbo, enjoy the ride! I have fond memories of my first few weeks of discovering the Shapeoko and CNC (and then for some reason the excitement never disappeared!)
There have been discussions on the forum about using router bits (example1, example2), so it’s not impossible, but as @luc.onthego said it requires precautions/specific feeds and speeds and toolpaths, so they are definitely not ideal for learning: my advice would be to buy a (small) starter pack of endmills (typically a couple of #201, #102, and #301, or maybe this new one), get familiar with using them, and once you master the CNC workflow and understand the constraints, then consider using your router bits.
If you’re going through the ebook, don’t hesitate to take notes on what’s unclear or missing in there and send me comments, I’ll do my best to take them into account for the next release.