It would be preferable to separate him - its just like us, if you were injured like that you'd want some space to be able to chill out in but hospital tanks are tricky - not a bad idea if you can have a sponge filter in the main tank for when you need to separate. You could also look at a tank divider, again you can get these online they are like a hard mesh panel with suction cups to attach to the glass, just to get him away from the others.
Unfortunately shops are not always the most reliable source of info - Bala Sharks for example can grow to 14 inches, they are a shoaling fish so need to be kept in big groups and they are also a migratory fish so in the wild swim huge distances through the year. By comparison the rest of your fish live pretty marginally against the river bank and some will never leave the same small creek or stream for their entire lives. Bala Sharks are super common in the hobby though so it is hard to stop people being sold them.
In terms of the decor holding bacteria, the vast majority of the bacteria is in your filter. Your tank has a decent number of cichlids but I dont think you are fully stocked bioload wise so I wouldnt worry about replacing it especially if you have the same concerns as us.
Cichlids are amazing fish to keep I kept a tank of them for 8/9 years and had ups and downs, lost pets that I really valued too early but also raised some fish to good ages. With anything like this its a case of learning when things go wrong, you've done fantastically to go through 3 years in the hobby without issues - I know my first three years were nothing like that
I'm not familiar with that brand of meds so can't comment too much, to be fair NT Labs have a good reputation so it could be worth keeping up with. Just for you to monitor over the next few days, healing will look like black edges to the wound and infections (bacterial or fungal) will look white and fluffy. There is a chance he will loose the eye but not 100%, he will be ok without it though as fish don't rely 100% on vision to see things they have sensory pits in their heads and the lateral line down the side.
But in short what to do over the next few days, keep the water as clean as possible, dim the lights a bit to keep them calm, try and get a divider to separate the injured fish from the others and either keep up with this medication or switch to the Esha, I'd probably ignore me and stick with what you have as swapping will take a few days and you will be better off sticking with what you have.
Wills