I haven't used TSM for SQL 2005, so my recommendations would be as follows:

1. Install TSM on a server with SQL 2005

or

2. Install SQL 2005 management tools on the SQL 2000 server

It is likely that TSM needs the 2005 tools since he has some hooks that will install into SSMS. It should be able to backup the 2000 databases - but just needs to have the 2005 stuff available.

And of course there is always the option of verifying with Tivoli.