1 brown knife would be fine in a breeder, they roughly reach 9", a BGK needs 125min as they can reach upwards of 18". I have kept bgk's dozens of different times. 1" a year is total crap. Haha if mine was growing at that rate if be looking into what I was doing wrong.
I'd say a senagal bichir (dragon eel) and a brown knife could work in the breeder. As they both reach the same max size 8-12". But you would have to scratch the leaf fish. All 3 would be a bit cramped and more then likely you would run into aggression problems.
Figure out what you want to be your centerpiece fish and build around that. Its been said hundreds of times on here "google is your friend" google and research what interests you then come asking for opinions/advise. I feel like to many times people who own fish in the topic want to just talk about them and their exsperience with them forgetting we are to advise on best way to house and care for fish, helping both owner and the wet pet to succeed.