In the Mixer view, you can easily adjust the volume and panning of individual instruments.

You can also export a MIDI file in such a way that an outside recording studio can match their instruments to each track and control mixing there.

Do what MIke Rosen said, control individual levels with FInale's mixer. Adrian's advice about using a recording studio's instruments is not practical. Finale uses human playback to send the correct midi data to the Garritan samples. If you switch your sound source to something else beside Garritan, then the midi file's commands won't have the correct effect on playback.