You're so lucky you were able to get those little shelves. I've been trying since day one with no luck. First the sales team, then the PM, and even the tile guys, as I happened to be there as they were starting the shower. No luck. Yours are perfect!
RH didnt care what I did in there - they just told me that I had to deal with the tile subcontractor...luckily the tiles guys were awesome. I still dont understand why shelves dont come standard with a tile shower. Even if you have a bench in your shower the shelves are great to have. Maybe one day in the future RH will change that! You can put marble shelves in after the fact, but definitely get a professional, a friend of ours did this themselves and cracked two tile during the installation.