Mine are in a 7.9 gallon tank right now out of necessity - but I hope to get my 20 gallon back up and running by the end of the summer to give them some more space and add to their numbers.
The footprint, as far as I'm aware, is more important than how many gallons it is exactly. That's the other problem with my 7.9gallong tank- it's basically just got a small square footprint and is slightly taller than it is long. If you have a 14 gallon that's longer than it is tall, and don't plan on keeping a lot in it (because kuhlis do best in groups, so you'd need a few of them), and fill it with lots of hiding spots, then I'd say it is fine.
Mine are very active, but seem to be happy leaving the safety of the bottom of the tank (it's just them and some shrimp in there, so they're not scared at all). Many other people find that their kuhlis, even at night, really just prefer to hide and don't need as much space.