I've never had Sofie, but based on the website, it says "fermented with wild yeasts", which I would take to mean brett. Do you know that they don't filter it, and/or that there's viable brett in the bottles?
Adding brett at 1.028 should provide plenty of food, so the question is more "do you want to see what their brett strains do to your quad?"
If you already have it in a 1 gallon carboy and plan on doing that, then it will probably be a fun experiment. Brett works slowly, and as such, you should probably make a starter to give it a head start (and also to make sure you're pitching viable yeast).
Brett normally takes a couple (to a few) months to get done, and especially if you're bottle-conditioning, give it plenty of time. You know, keep checking gravity samples after a couple of months to make sure it's not still going.
As a side note (*and entirely my opinion*) -- I wouldn't even bother moving it to a new, one gallon container unless there's tons of hop gunk etc. in the primary. Just add the brett and let it do it's thing. A couple months in a one gallon carboy shouldn't result in much in the way of off flavors.