Did you bid the job yet Clay?
It would be interesting to know what the architect intended for his spec.
We used to do silicone grout all the time on bath walls and fiberglass pools & spas. Did tons of it. It won't shrink at all and is relatively easy to clean up. Having said that - I can't see it being a good choice for grout on commercial flooring.
If silicone rubber is indeed the spec for the grout - there are a couple of ways it can be installed ( 1/8" maximum joint width).
1) caulk joints using air or battery powered caulking gun using care to make full contact with the sides of the joint leaving them full, striking the joint with the tip of the tube. Apply soapy water (approx 4 to 1 water to dish soap). Further strike joints (with finger or thumb) as needed for consistency disposing of excess frequently. Remove any remaining residue with wet cheesecloth. Smooth any remaining marks in joint with finger. Clean after 24 hours.
It's faster than it sounds.
2) dump silicone onto floor. Smear into joints using a regular float. I like a wall float as it smears cleanly. Clean using soapy water and reticulated foam
sponges ( your local foam/ plastics supplier can usually cut these for you). Dispose of sponges as they fill with silicone. Smooth joints with fingers/ thumbs. Clean after 24 hours.
This process should take a little longer than using an epoxy grout so charge accordingly.
The only other thing to add is - better you than me