TCAS has its own limits for RAs during approach and take off.
Below 1500ft AGL no INCREASE DESCENT RA's will be issued, below 1100ft AGL no DESCENT RA's will be issued and RA's alltogether are inhibited below around 1000ft AGL as the TCAS system switches to TA ONLY status.
My current TCAS manual doesn't have any information about this, however an older version told us that TCAS is able to track up to 127 targets at once, which does not mean that they are displayed.
TCAS cannot see true Mode A only transponder equipped targets, however it can see Mode C both with and without altitude reporting enabled. There are not many Mode A transponders around, most are at least Mode C and Mode S is becoming quite common now that it is required in many european airspaces, that might differ outside of europe of course.
If a Mode C target with altitude reporting is in the way TCAS will calculate that into its solution taking into account that no coordinated solution is possible. It will generate a proximity alert and a TRAFFIC TRAFFIC callout for Mode C targets without altitude reporting, but cannot take them into account for any RA solution as it doesn't know where exactly they are.