Clown knifes and Black ghost knifes are two different fish. Do you mean the black one? Thats the Black Ghost Knife. They are beautiful and fascinating fish. I've only had mine for about a month or so and just totally love her!!
It would less risky for you to get them onto pellets and fresh or frozen meaty foods. Feeding feeders is always risky.
I myself feed frozen silverside fish instead of feeders. And live Ghost Shrimp.
I also feed frozen:
mysis shrimp, bloodworms, krill, beefheart, brine shrimp(as a treat as it isn't very nutrtious) fresh grocery store shrimp.
And for pellets:
Hikari sinking carnivore pellets, New life spectrum, Omega One shrimp pellets, Tetramin variety pellets for bottom feeders.
But they also love frozen peas and some greens too.
Not sure on keeping more than one BGK fish together. I will let others answer that for you as I've never done it but hear it can be done.