As far as I know, custom bikes are not allowed in the peloton, to discourage crazy and expensive competition for the next great breakthrough---similar reason why the minimum weight is so high. As far as I know, riders have to actually ride models offered by the makes---though all the accessories can be whatever 9like the custom stems mentioned above.) I believe they all use off-the-shelf drivetrains to keep costs down .... though they can buy anything on the market, there is so much available that getting hand-built one-off gear (and then spares) would eat up the budget.
That's pretty much how the UCI wants it----Not a competition of Bike versus Bike, but instead .... which rider has the best pharmacist.
