It is recommended to select TCAS off while on ground (and not on a runway) because otherwise mode A and C will also be active and lots of returns from many aircraft on ground in close proximity creates problems for ATC secondary radar.
Can you back that up with an FCOM reference? I haven't found anything that supports that view, although it is expressed by some trainers.
Which transponder mode is used depends on the transponder mode switch, which should be in AUTO on the ground. In that case it transmits only in Mode S with the ground flag set, which excludes it from any TCAS and unwanted ATC display. Unlike ON which does indeed switches to Mode S/C regardless if on the ground or not and will usually lead to very fast complaints by your friendly local ATCO.
Setting TCAS to TA/RA does not override the transponder mode selector, try it out next time you're in the aircraft.