TCAS uses Transponder Mode C (Altitude), which is ALWAYS referenced to 1013mb. This is because, for both TCAS and ground SSR systems, comparisons between levels of different aircraft need always to be referenced to the same datum.
Therefore, the only way that changing an altimeter subscale setting can affect TCAS is if, by varying the subscale, the aircraft actually reacts, i.e. it climbs or descends because ALT CAPTURE is engaged to that altimeter (ADC) system. In this case, the actual change or trend, if it is significant, may prompt a TA or RA.
However, if the AP is not coupled to the altimeter (ADC) in question, or if ALT HOLD or manometric locks are engaged, then TCAS cannot possibly be affected by changing a subscale.
JD
