BGK's can be territorial towards other fish if the tank's not large enough and other fish are constantly swimming around his private territory so to speak.
Also as they grow they will tend to eat smaller fish, so those Danios may end up missing at some point.
If given the space and the water quality/maintenance is kept up and they are provided with a variety of foods then they should grow pretty quick. One of my customers has a BGK that is a little more than 14 inches and has had him for about a year and a half. The BGK was 6 1/2 inches when he bought him.
Sure you could keep a BGK in a 55 gallon for a little while, but as soon as it reaches about 10 inches I would move it to a bigger tank as a 55 gallon is only about 12 inches wide....doesn't give the BGK much room to move and turn around if you ask me....length wise he would be okay but that's only if he just sits there, but he does swim around and get very active at night, so those measurements are going to be slightly restrictive.
The way I see it (and this may just be me) why bother getting a fish if you know how big it's going to get but only keep it in a tank that's barely big enough for him? When people buy fish they need to think about their health and life in the long term. ONLY if you ARE going to get a BIGGER tank should one even consider getting a fish that's going to grow that large...it's for the fish's benefit. Just think of someone having to live in their closet or bathroom their entire life....to me that would be boring not to mention unhealthy....you would get lazy and just sit around, eating and drinking etc and then no way of working all of that off to stay healthy.
The bottom line is this. Don't put a fish that you know is going to get big into a tank that's not suited for it's needs in the long run. It's not right. How about looking into a different type of knife fish. I know there are certain African Knifes that reach a max of about 8 inches or so. They would be better suited for a 55 gallon tank and you wouldn't have to worry about them outgrowing it. Just a suggestion
To sum it up LoL
BGK's can be territorial towards other fish.
They will eat smaller fish as they grow.
Just not fair to the BGK to house him in the bare minimum.
Hope that answered a few questions you had

Good Luck! Keep Asking questions too!!! We're here to help! :bday: