Hello Gaurav
Split the stack and then connect the systems to the Solman SLD/LMDB
Create separate stacks for Java and ABAP for the EHP7 update.
I would update the ABAP and Java at the same time.
Regarding the other systems in the landscape it is mandatory to upgrade the connected systems to the latest supported release. You will need to upgrade those system if they are below NetWeaver 7.30 release.
The supported combination of ECC and CRM is provided in this SAP note
1406261 - SAP for Utilities: Release combination CRM and ECC
You can make use of the UDA tool to know the dependencies involved.
You may read this note to know more.
1951805 - Adjusted Version Interoperability for Business Suite 7i2013
Regards
RB