Bala sharks will be fine as they are very fast swimmers. I have a very large tank with 3 Red Necked Severums which also houses two young Silver sharks. The Severums have now grown to over four inches and one is going for the the Silver Sharks every now and again but isn't coming close to getting them. My advice would be to buy a group of small Severums and grow them on buy which time the Bala Sharks will have alos grown a bit.
Other good companions for Red Necked Severums are
All the varieties of Silver Dollars, Rainbow fish, Angel Fish, Geo and GymnoGeophagus species, Denison Barbs, Clown loach.
Check out Magyver.co.uk for a few pics of my fish. Only a few as i am still putting the site together.