I have three sharks 2 lemons and one black tip. The biggest thing is that you need to feed sharks, and everyone puts the money into the tank, but their filtration system is usually sorry. Therefore their water parameters are bad, and the sharks suffer.
Sharks are pretty easy to keep with adequate filtration, food costs are high. $40 weekly in fish for my tank alone.
What do you plan to do when its too big?
Those are some of the things to consider, as for as additions to the tank, if the sharks are fed, most good sized fish are fine.
If you are looking at keeping small sharks, you can probably keep them in a 450 gallon tank, but make sure you spend the money for filtration.
I made a commitment to my wife, 23 years ago, she was easier then the sharks.