Hi,
As far as Unix and linux backups are concerned, "legacy agent" means the agent which is avalable for a "free" download.
The agent doesnt have the functionality of the RALUS agent-like backing up mount points and backups with legacy agent are known to be slower than those performed with RALUs agent.
Legacy agent has its own software compatibility, which can be referred to here:
Symantec Backup Exec 10d (10.1) for Windows Servers - Software Compatibility List (SCL)
http://support.veritas.com/docs/278254
Symantec Backup Exec 11d (11.0) for Windows Servers - Software Compatibility List (SCL)
http://support.veritas.com/docs/284493
VERITAS Backup Exec (tm) 10.0 for Windows Servers - Agent for UNIX version 5.046 (Legacy Agent)
http://support.veritas.com/docs/269803
Regards
Padmaja