So I own a 1999 MCI 102-DL3 w/ a Detroit Series 60. My dad also owns a 1993 MCI 102-C3 w/ a custom Cummins M11 engine swap. We're bus nuts!
I bought a Bluefire for my bus, and it works seamlessly! The only thing to note is that I have the 6-pin connector, so I had to buy an adapter to plug in the 9-pin Bluefire. It also works great on my dad's bus, although that's irrelevant because the engine swap changed all of the engine electrical.
The reason they work is that most MCIs of that era, while they mostly use 24V, have a 12V engine computer. If your ScanGuage works then you should be safe with a Bluefire, but Bluefire support should be able to tell you for sure.
BTW, having all the information that a Bluefire gives is super handy! I set up an 8" tablet to show everything from a tachometer to my retarder temperature, then mounted it on my A-Pillar. It's expensive, but well worth the money!