Personally I wouldn't do it. I'd be worried the bichir would try eating the slender bgk... And aren't brchir from Africa? Its not usually a good idea to mix contents of you can help it.
Although I've never kept bichr so I don't actually know if one would try to eat a bgk...
Disclaimer: I do not know everything, nor to I claim to. Always do your own research and always cross reference!
Depends on things like tank size, hiding spots, the aggression level of your particular poly.
Since the bgk is so slender at that size even a delhezi could attempt to eat it. If it were me I would want to start with a little bigger knife if possible.
Can be mixed but the bgk should be bigger. The ones you have can be mixed at that size. I've had the two at that size prior without issue. Dmd is right!
Can vary as far as outcome and nothing wrong with waiting till the bgk gets much bigger.
I've kept both in the same tank before with no issues. As long as there are plenty of hiding space and they are well fed, there isn't anything to worry about.