I keep mine with rays with no problem. However, years ago (an some of you may remember) I had a small hystrix mysteriously get a chunk missing out of her disc. The knife was the only thing that could have done it...though it was a rarer species knife that I can't even remember the name of...but very close to a bgk.
They can sometimes be territorial but with all the space you have, you shouldn't have a problem.
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could be an aba aba - they are really aggressive. BGK are modest and calm specie - im more afraid for them to get eaten rather than causing harm to the ray.