IIRC, the 6100 shipped with no ROM SIMM (ie, ROM is on the logic board). Try booting it and see. If I am wrong, the 7100 ROM should work.
Many Accelerators and video cards to go with them, including a Sonnet Crescendo 500MHz G3 card
The one card goes into a slot, and the other goes into a PDS/Nubus? slot. They are attached via an orange ribbon.
Is the ribbon cable permanently attached, or socketed? In the 6100, the ribbon cable is not required - the video card plugs straight into the accelerator. In the 7100 or 8100, the video card would collide with other components, so the ribbon cable allows you to move it over to a Nubus slot space.
The second card is almost certainly a video card - Apple AV with a Mac video port and two S-Video ports, or HPV, with only the Mac video port. When the accelerator is not present, this goes directly into the PDS slot.
If you post a picture, we can probably ID the whole thing for sure. In fact, throw us some pics of anything you're not sure about
If you do have the socketed ribbon cable, that would let you mix & match accelerators & video cards in the 7100