i currently have a black ghost knife, delhezi and an ornate bichir in a 240 gallon tank. Size is key. as long as one can't fit in the mouth of the other you should be fine. The only issue i have is that the BGK and the delhezi react to the food a lot faster than the ornate so i have to pretty much drop the pellets on the ornates head to make sure he gets his share.
I assume you are referring to a senegal which is supposed to have a similar activity level to the delhezi so i dont think you would have this issue at all.
a BGK is supposed to get to 24 inches or so, but they are extremely slow growers. i have had mine almost a year now and he has only grown about 2 inches. a 75 gallon might be too small for a full grown one, but it would probably take years to get there. Not that i am advising getting a fish that will eventually out grow your tank but if you do, you may need to have upgrading plans in the future.
