I'm a big fan of big schools of smaller fish, personally.
IMO you need to rehome the Bala shark sooner rather than later, anyways. They're schooling fish, and even more nervous and prone to slamming into tank glass when kept alone than when in schools (and they're still bad about it even when in a school... just not a fish that should be sold in the hobby IMO).
With the Bala shark rehomed it would open up the doors to many, many more stocking options.