I had a Dewalt DW616 router I got from a hardware going out of business and I never took it out of the box. Would It work on the Mini Mill (meaning would it fit in the bracket)?
No, these are the same diameter as the 618s which I believe are around 85mm. The included mount is 71mm. Making your own mounts is really not that difficult though. OX HD
Or, there are these. I bought the 65mm mount for my Makita and it is great. Shop 85 mm spindle mount online Gallery - Buy 85 mm spindle mount for unbeatable low prices on AliExpress.com. They are 150mm wide, so you either need a plate that big, or you may need to make a plate to mount it on which can mount on your z axis plate.
A followup question since I am new to CNC machining: is there an appreciable quality/usability difference between using a router motor versus a spindle? They seem to be basically the same thing...