It's recommended that players wait to extensively modify Ada before heading into the final mission to track down the Mechanist.
It should be noted that robots (including Jezebel) can in fact be used as provisioners and assigned to tasks, making them a good way to build up and protect settlements. Jezebel can also be modified, but at this point does not appear to be available as a companion, so wasting resources on her is not recommended.
Ada and Curie can both be modified to increase their protection, armament, and other capabilities.
