The bostik grout is perfectly fine over the kerdi. The grout joint size depends on the flatness of the tile, the flatness of the substrate, the caliber of the tile, and the skill of the the installer. You can always test the grout over the tile for scratching, but we've never had a problem.

Love the florida tile......what series Florida tile?

weve installed a fair amount of their Progetto line.

no grout will scratch that awesome material...

something else to consider is the edge profile of the tile, with floridas progetto the edge is beveled and a 1/8" Spacing of the tile actually reveals 3/16 "+ grout joint at the surface.

Thank you. The series is Gallant and their listello, Plaza Bianco is gorgeous with a little bit of sparkle

I dislike urethane grout strongly. I used it once before (in my own kitchen) and will never touch it again.

Kerdi is another product that I used to use and have since found much nicer products with far less restrictive setting material choices.

I would not use Kerdi unless you have access to Ardex Setting Materials. Then I would be choosing between Ardex's cementious grout or their epoxy.

Sounds like your installer will be mixing and matching products from at least three suppliers to build your shower.

Why not switch out the waterproofing to Laticrete or Ardex's system and then use that company's setting material and thin-set. Then all the products work nicely together.

Call the maker of the tile. Ask them what thin-set you should use. Then look up what;s required by Schluter. You will most likely find the two specs conflict.

Disagree wholeheartedly about urethane grouts. That's what most (good) installers around here have shifted to for a reason. No callbacks. The color stays consistent, like it's supposd to. It doesn't need sealing. It wears like iron. It's much easier to cleanup than epoxy. And it's more forgiving to the hacms that tend to rule the business around here. It elevates their work beyond their skill level a bit, and encourages them just a tiny bit to learn more abut new products and methods. Win/win.

Only trying something once, and not trying a couple of different brands to find a fit, is cutting your nose off to spite your face. Try TEC Power Grout. It's what I did for my own personal project, and it's great.

five years ago I used that grout in my shower over Kerdi. I have 9x18 porcelain tiles with 1/8" joints and some glass mosaic on the walls. and 1x2 glazed porcelain mosaic on the shower floor. no tiles were scratched and it has worked perfectly for me. the contractor even used it on the change of planes and I haven't had any cracking. i love not having any caulk. I know that is not recommended on change of planes, but it has worked for me. I squeegee and wipe it down after every shower and never get any mildew.

if the contractor uses that grout regularly and is comfortable with the installation, I would recommend it. my contractor had never used it and complained some about the difficulty of install.