Hi Sreekanth,
You are already trying to install MaxDB 7.8 in the same server. The issue is that you want to keep the existing databases without any upgrade. it is best to make them consistent.
According to the installation procedure for Solman 7.1 with MaxDB 7.8, you need to first instal 7.7 and then upgrade to 7.8.
Please note that the programs and libraries in program path (sapdb/programs) will be updated and used by all database instances. See: http://help.sap.com/saphelp_nw2004s/helpdata/en/44/1549d945f71a74e10000000a1553f6/frameset.htm
Based on the following link the global listener is also downward compatible and should not cause any issues to have the 7.6 databases
http://maxdb.sap.com/doc/7_8/45/376baca05f6bf1e10000000a1553f6/content.htm
I haven't had different release of MaxDB on one server but I think it is possible.
It was also best to install these systems on separate server. I guess if you need to restart one system then you need to restart all. see: http://scn.sap.com/thread/690627
I hope this helps,
Regards,
Shahab