Hi Debasis,
All your doubts are already clarified in above note, Note 69455 - Servicetools for Applications ST-A/PI (ST14, RTCCTOOL, ST12).
1. See Else part of Point "3. Upgrade behavior" in the note 69455.
2. The only prerequisite for ST-A/PI is a minimum SPAM/SAINT level
- 36 for SAP basis release 4.0B-4.6D
- 20 for basis >= 700
- 16 for SAP basis release 6.10-6.40.
3. There is no any known problem during ST-A/PI 01Q_731 add on installation through t-code saint.
I hope your all doubts are now cleared.
Regards,
Amit