A FortiGate-7000 will continue to operate even if an FIM or FPM fails or is removed. If an FPM fails, sessions being processed by that FPM fail and must be restarted. All sessions are load balanced to the remaining FPMs.
If an FIM fails, the other FIM will continue to operate and will become the config-sync master. However, traffic received or sent by the interfaces of failed FIM will be lost.
You can use LACP or redundant interfaces to connect interfaces of both FIMs to the same network. In this way, if one of the FIMs fails, traffic will continue to be received by the other FIM.