vCenter 7 should support ESXi 6.5 hosts however you have to be very careful if you upgrade the VM compatibility level otherwise you won;t be able to run those VM's on a 6.5 cluster.
I would strongly discourage using 6.5 for DR as it's out of support and will cause problems with compatibility in the future especially when 7.0 goes EOL on 02 Apr 2025.
Best practice would be running the same version of vSphere on both prod & DR and I would also recommend having a separate vCenter for DR
