A tube of brett isn't too much. Actually, the cell count in the brett packs/vials is much lower than ale & lager strains. You might as well toss it in right away. Brett C can be a bit of a slow-poke sometimes, so that'll give it more time to do its work.
The only time I would wait is until after primary fermentation is done, but you're already well beyond that point with this beer anyway. Good luck!