I have a single chalceus and I love it. In your tank one would be best rather than trying a group.
An oscar would work also, I've kept the oscar/severum combo before with no problems. What might cause you trouble is getting them all at the right size. BGK and chalceus don't grow particularly quick, especially compared to an oscar. You'd want to get the knife and/or chalceus bigger than the oscar to start, but not so much bigger that they (the knife and bichir in particular) could eat it.
An oscar would work also, I've kept the oscar/severum combo before with no problems. What might cause you trouble is getting them all at the right size. BGK and chalceus don't grow particularly quick, especially compared to an oscar. You'd want to get the knife and/or chalceus bigger than the oscar to start, but not so much bigger that they (the knife and bichir in particular) could eat it.