A TA or RA is only triggered if an intruder enters a specific 'airvolume' around your aircraft. The size of this volume is NOT fixed. It depends on airspeed and flightpath of intruder and your aircraft.
The radius of the volumes are expressed in 'closest point of approach' (CPA) and has noting to do with the altimeter settings of both aircraft.
In my opinion, no problem to select QNH or FL asap.
Ciao
The Snake