No suggestions for centerpiece fish, but I did want to alert you to a few things, if you didn't already know.
Did you know that bala will grow to around 14 inches, needs a 100 gallons tank, and needs to be in groups of 4 or more? Once grown, it'll also start eating tankmates that are smaller than it....
Discus get to 8 to 10 inches, need to be in groups of at least four, and required 10 gallons per fish when grown. (Minimum tank size is 55 gallons.) Anything less than four will create fighting.
The loaches and the ottos should be in groups of at least 5 or more to be comfortable. There is a dramatic change in personality when they're able to shoal with the correct number of buddies!
Red-tail sharks can also be very aggressive, but it does depend on the shark's personality. That might be problem if you decide to add other fish.
I know, you go to the fish store and they have so many cool types of fish, but please keep in mind that the employees and the tags on the tank are wrong 99.9% of the time! The good thing is they'll often take back fish (which you might want to consider with the bala and maybe the red-tailed shark as well). There's a great research area here called the Fish Index (on the front page of the forums), and of course if you need anything clarified, the people here are great!