Hi Reza,
If you are performing update of ST-A/PI then yes you can go-ahead applying by using SPAM transaction. If you are installing ST-A/PI then you should use SAINT transaction.
Also make sure you have update SPAM/SAINT patch to latest, so that you dont get issues while applying ST-A/PI. ST-A/PI is the small patch and you should not get any issues if SPAM/SAINT is latest.
We are here to help you if you again stuck in updating the same.
All the Best..
Best Regards
Kishore