I use to own a BGK.
In my experience, once they reach about 6-8 inches long, it is time to house it by itself. Yes, there are possible tankmates, who must be non-aggressive and much larger than the BGK's mouth, but your tank size to support this combination would need to be very large, I'd say a minimum of 125 gallons.
BGK are usually pretty slow growers. It takes on average of about a year to reach 6-8 inches, and another year after that to reach it's expected minimum size of a foot/12 inches. So for the first year, any docile non-threatening fish can be kept with them, and if the BGK is kept fed with proper nutrition through frozen foods like bloodworms and brine shrimp and the like, the less likely the BGK will eat other fish too soon. Its usually during that second year of growth that the eating of tankmates can happen, but as I said, a well fed BGK is less likely to eat his tankmates, especially if the tankmates arent tiny things like guppies and neons.
BGK arent aggressive with other fish, just predatory by nature. So they should not be kept with any other fish that can be aggressive. I have seen BGK babies successfully kept with female bettas while in their first growth year without issues.
I personally do not recommend ANY bottom dwellers with the BGK. BGK are easily trained to come to the top of the tank to feed from your hands. When young they love to sleep upside down like an arrow inside of plants, and they always need appropriate hiding spots with no sharp edges since they are scaleless. Any meds or plant ferts used in the tank should be used extremely carefully as BGK are super sensitive to these things, comparable to the sensitivity of loaches being scaleless as well. And never put BGK with any other knife species or eel or any species that uses any electrical/sonar to find food and see.
With BGK it is wise to keep the tank set up the same, they dont tend to take well to too much change. Some will refuse to eat or come out of hiding when there are tankmates that stress them, which when young can be a shoal of danios or tiger barbs.
BGK have poor eyesight, close to blind, they use their sonar to locate food and locations of hiding spots and threats and targets.
These guys not only get long, but they get thick around, so they cant easily turn in small tanks like more snake-like fish that looks similar, so a wide tank is important when the start to size up.
A BGK without tankmates when grown, should be in a 75+ gallon tank, the larger the better.