Not a pro musician, but here goes...
The Triton is a Midi controller/sequencer with a built-in sound generator. Garageband is not a sequencer or controller, but it too, has built in sound generators. The main use for GarageBand seems to be recording.
Since the Triton has midi out, you can play the GarageBand insturments using the Triton. Or you can plug the Triton's audio out into your Mac and let Garageband record the audio. Garageband cannot control the Triton since Garageband doesn't have midi out.
So you can take advantage of Garageband's loop capabilities and the Triton's live performance and sequencer capabilities at the same time, I think.
For composing, I think they complement each other. For performing, I think they complement each other too. And for recording, I think Garageband is the only alternative, since the Triton doesn't record.
And the midi interface you asked about should work fine.
Chris