I think sharks need a minimum of 30 gallons (this is for red tailed black sharks and red finned black sharks, the most common of the sharks), but i couldn't find any info verifying this. With the sharks, you need to watch out for agression issues. Because of this, you only want one RTBS or RFBS per tank. Bala sharks grow to be enormous so they are definately out of the question for your tank.
Instead everything i saw said atleast a 3 foot long tank, so you can measure and be the judge of that. As for the lights, I usually leave my lights on close to 12 hours, just to simulate the wild, but it is largely up to you. Do make sure that they get some dark time though, because fish do sleep at night.
Have you read up on the cycling process and do you own a test kit to check your water for ammonia or nitrite? This can be a very stressful time for fish, so you definatley don't want to add any more until your ammonia and nitrite levels have dropped to zero.
