get em, and if they get to big just return em to your fish shop. Some people like buying fish that aren't babies.
I had a black ghost knife and it didn't eat to much and didn't grow to much over a 2 year period (till my piranha got tired of it) it was about 10" long and I rarely see them longer than that in pet stores.
Clown knifes can get enormous though, I saw a 2 footer at one pet store out here, was insance looking....
Either way, ghost knifes are boring as hell, they only really come out at night, I wouldn't get one ever again.
And those elephant things are just ugly....