What is your pH and GH? KH is just a indicator of how much buffering capacity your water has. So keeping it at 5 or 6 ensures your pH won't swing up and down but stay stable-which is a good thing. The lower the KH the more your pH will swing and can kill your fish.
Yes pH and GH are more relevant but in any case Threadfins are highly adaptable given the large seasonal fluctuations they experience in the wild. They should be fine in water parameters suitable for tetras.