You do not need to upgrade the Adaptive Server containing the RSSD to version 15.0 or later, before you can install Replication Server 15.5
Disregard the following text in “Upgrading and mixed-version requirements,” in Chapter 1, “Before You Begin”:
You must upgrade the Adaptive Server containing the RSSD to version 15.0 or later, before you can install Replication Server 15.5.