From the release notes: http://seer.support.veritas.com/docs/278254.htm
"The Backup Exec 10d (10.1) Remote Agent for Windows and NetWare Servers is strongly recommended for use with Backup Exec 10d (10.1) for Windows Servers; the Backup Exec 10.0 Remote Agent is also compatible for the purpose of rolling upgrades with the exception of any caveats listed in the notes. The Backup Exec 9.x Remote Agent for Windows is not supported and not recommended for use with Backup Exec 10d for Windows Media Servers."
So, while it's not saying 9.1 remote agents definitely won't work, you may well run in to a few glitches if you do try running 9.1 Remote Agents with a 10d Media server.