Look up the number of the manufacturer and call them, and ask them, if it will scratch marble. Yes, I guess, you need to put some trust into your installer, but, if you read these forums, a alot, you will also know, that installers, do not always know best, and some in fact, seem to know almost nothing.

T.F.W. is correct. Real pros choose their products based on performance and specifications for the particular application and experience. We don't use Tec but use Laticrete Select or Permacolor with marble installs (our own preference). And we do lots of marble installs. Installer needs to be comfortable with the product and able to follow manufacturers recommendations as TFW points out.

I also prefer Permacolor. But that said, there's nothing wrong with TEC Powergrout. Pretty much TEC's version of Permacolor. The sand is alot finer than normal sand aggregate, and so can be used in smaller grout joints. I also don't have a problem using the fine sand grouts on even polished marble, much less honed. Bottom line, though, your installer is responsible for the end result. I don't see anything wrong with what he wants to do, from what you've told us to date.