Flex scheduling and fixed scheduling simultaneously in our units

I am an ICU RN who is helping formalize our flex-schedule (self-schedule or variable schedule) option along side our fixed schedule choice for nurses in critical care. We have 3 units (ICU, IMCU and CCU) who all do separate scheduling but look to each other for models. For the last year in ICU, we have offered flex-scheduling to those nurses who would prefer it to a fixed schedule while keeping fixed schedules as an option. At this point, with about 50 full and part time day shift RN's, we have about 1/4 of our nurses on flex schedule. As a union hospital, we have followed the contract guidelines, but are concerned about nurses with the fixed schedule grieving the scheduling process as this new flex option grows in participants.

Does anyone else out there offer this? Are we crazy to do both types of scheduling? How have other units put guidelines in place to promote equity and decision making?

So far night shift has decided to stay with fixed schedules only for sleep regularity. Other than that, our actual flex scheduling is going great. Would love to hear from others who are doing this creative schedule combo!