GSX doesn't use or call or interacts with FSUIPC, it only makes direct Simconnect connections to the simulator, FSUIPC will make its own separate connection, so there shouldn't be any interference between them.
I'm not familiar with any of these products but, since you say one failed to detect flight start, it might be helpful to ask the developer how a flight start is detected by that program, so we might possibly have some idea of a possible conflict.