Salt will slightly decrease the amount of O2 that water can hold.

I don't really know all that much about hardness but I kinda doubt it would have the same effect as salt. You want your hardness to be appropriate to your fish and the buffering capacity to protect you from pH swings and your pH to be as close to neutral as is reasonable for your system and water situation. Stability being more important than any particular number I believe. Drastic sudden swings are usually harder on fish when it comes to pH. A really low pH will stop one part of the nitrogen cycle while really high pH may stall another part of the cycle. If stability can be achieved, you will likely eventually find a batch of bacteria that can work in your system.

I suppose in a quarantine or hospital system, you might want to have the ability to adjust the pH, temp and salinity but it might be easier and more appropriate to acclimatise new fish to your normal system conditions rather than trying to adjust your system if it is stable. pH bouncing and sudden temp changes are really bad for fish.

Yeh, I'm kind of worried that although my hospital tank is going to have good conditions, the fact that they are different to the main tank is going to shock the fish and do more harm than good.

The trick would be to have to ability to bring the hospital tank conditions into accordance with your main system slowly so that it would be a good thing rather than a bad one. Otherwise it would be best to have the hospital system as similar in conditions to the main system as possible.

For me I think that will mean using a similar mix of grow bed media to give the same buffering and hardness that the main system has. (The shells in my media seem to keep the system quite solidly at a pH of 7.6.)