Your question is bit confusing. There is no such thing as MSSQL ASE 15.7.
ASE is available on windows platform BUT its is not same as MSSQL.
If replication is from ASE to ASE you can use RS 12.6.
The flow is : ASE ==> RS ==> ASE
If replication is from ASE to MSSQL then you will need a companion product called DirectConnect along with RS.
The flow is : ASE ==> ( RS + DC ) ==> MSSQL (using ODBC)
More details in documents on heterogeneous replication.
Just curious, if you already have RS license why not use the latest RS 15.7 ?
Avinash