I have a similar setup (keezer), and i really want to start kegging a wider variety of drinks, but I'm not going to make a whole 5 gallon batch of tepache. Putting 1 gallon of liquid into a 5 gallon keg isn't ideal cause you either have a lot of oxygen in the headspace, or have to waste a lot of co2 purging.

Unfortunately, 1 gallon kegs are way more expensive than 5 gallon kegs.