Hi Sudheer:
First you need to make sure that your Plesk database version is also 7.1.5:
> mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e"select val from misc where param='version'"
If the Plesk psa database version is still also 7.1.5 - then it is OK, and you can proceed with removing other installations almost like you proposed:
> rpm -e --noscripts --nodeps --justdb psa-7.1.7-rh9.build71050228.12 psa-7.5.4-rh9.build75050824.12
(all those keys --noscripts --nodeps --justdb are required to keep other Plesk files, and avoid the dependencies failure).
Hope this helps!