well I also have a black shark, and have done a lot of research on them, and have also talked to somebody with expierence, and some sites say 9-12", I have talked to a guy that has had one for years and it hasnt grown over 9" for years, so with this and other information I have gathered I have found that in the wild they may hit 22" but in captivity it is extremely unlikely, despite what everybody says.
As for the Kuhli Loaches I am unsure just for the reason they may become lunch as mentioned even with the sharks at 9-12",, but that is just the only reason I would figure it wouldnt work.
I did find upon research that one site did say it would work...
try googling it
Off to bed..
Murph