The 7100/66 is a Nubus machine, that G3 will only work in Nubus machines in the PDS slot normally used for video cards.

I have a 7100/66 with a Newertech G3-250/1MB cache that has 128MB of RAM and a 9GB HD, OS 9.1 took forever to load on it, and wasn't that snappy. OS 8.1 runs much better. It has the attached PDS video passthrough which is nice since the 7100 has that funky video connector on the motherboard.