You can mix and match Shimano 11 speed components with no problem. My 2015 Allez Comp Race came stock with Axis (Specialized house brand) brakes and I upgraded them to 5800 105 for a noticeable improvement. The crankset and BB on the Roubaix you're considering are excellent as well, you won't have any problems with creaking.
__________________
Regards,
Chuck
Demain, on roule!