Hello! Here is a website where you can enter the measurements of your aquarium and find out for sure what size you have. You need to know this so you will know how much dechlor, etc. to use when you do water changes. Aquarium and Fish Tank Volume Calculator
I hate to have to say this but, if you are willing to take the Bala Sharks back, that would be best for you and them. Here's why... they are not playing chase, the dominant one is after the other to show he's the boss and will literally wear that poor guy out, sometimes to it's death. I would imagine you bought them from a tank of several Bala's, they are schooling fish and need to be with others of their own kind and this lessens the aggression. You are right, they grow very big so you will be faced with this issue either way before very long and have to get a much larger tank, or re-home them.
Once you decide what to do, you have lots of options of fish that will work comfortably with your Bristlenose Pleco
and the size tank you have. I advise you to look at websites and decide if you want semi-aggressive or community fish and go from there :)
It is always exciting to start a new tank and you will have lots of fun, be sure to show us pics as you progress!!