Actually, I would recommend that you plug the AIO pump into that CPU_OPT header. Then be sure to set it to use fixed max speed at all times. Why that particular header? To some extent, I'm just hoping for something here. You have not told us what mobo you have, but most mobo manuals do not disclose what I'm looking for anyway. On all mobos, the CPU_FAN header pays special attention to the fan speed signal at that header for detection of FAILURE of the CPU fan. So even if that is really a PUMP on the CPU, failure detection of the pump is just as important. Now in your case, the recommendation is to plug the rad FANS into the CPU_FAN header so their speeds are governed by that header, using the CPU chip's internal temperature sensor for guidance. The PUMP should be on the CPU_OPT header set to max speed. I am hoping that the mobo actually does monitor the CPU_OPT header speed signal for failure, just as it does on the CPU_FAN header.
Now, to be sure of failure monitoring, you could reverse that. Put the AIO Pump on the CPU_FAN header and set it for fixed max speed, and it WILL be monitored for pump FAILURE there. In an AIO system, pump failure is the most critical item for failure monitoring. Then connect the rad fans to the CPU_OPT header and set it to its normal automatic fan control, ensuring IF there's a choice that it uses the CPU internal temp sensor, and not one on the mobo. Most CPU_OPT headers can do this just fine. It is the dedicated PUMP headers on some mobos that cannot control the speeds of their devices. IF the CPU_OPT header does monitor its load for fan failure, that's good; if it does not, other CPU overheating protection systems will be sufficient even if one or both rad fans fail, as long as the pump is still working..