If your mini is running OS X, for this discussion and purpose, it is a "Mac." You can use it to make the USB key. All you need is an external dvd drive and an 8 gig key.
If you don't have an external dvd drive, I would recommend an XBOX 360 add on HDDVD drive, Fry's has them for $28 or so, as the format is dead. Works great with the mini. It works with OS X post install also.
NBM/NBI were written because SL required a new install method. I would recommend, based on my experience, as well as others who have posted here, in doing a full fresh install of SL, not just upgrading your 10.5 install.
The SL upgrade disc will allow you to preform a full install. It seems to work better, with fewer issues than an upgrade of an existing install.