SRM should have its own Site Recovery Manager license key.
Based of this matrix here for SRM 8.1 :
https://docs.vmware.com/en/Site-Recovery-Manager/8.1/rn/srm-compat-matrix-8-1.html
Downgrading from vSphere Enterprise to Standard should work (but not for advanced).
The only things that you need to check is the inter-ops matrix between your SRM version and your Esxi 6.x version.
https://interopmatrix.vmware.com/Interoperability
And also, if check if this is the same for your SRM version if you are lower than 8.1
