You'll get loads of answers on this. Most with no scientific proof. One thing when I had my in ground pond I did not use chemicals , medications... all natural ! Nature does not have a drug store and nature does very well with out one. Prevention is key. See you found out about quarantine tanks !!! A healthy RES is very hardy. Now one with poor diet and issues that's another story. Yes they can , bad bacteria is bad bacteria.
Large ponds are hard to dose , some say little extra can't hurt but it does. Melafix over dose can hurt there eyes. As far as bifuran its ok with turtles , frogs , snails ... and was great for slime coat on koi. Most over stock there ponds. A good quality female koi can reach 36" if well taken care of , being 1000 gallons for her and I used 500 gallons for each male. You will hear other , then it's your call ! Then you will always have someone say one more won't hurt. I call that the "one more won't hurt syndrome" for fish and or medications !
Salt now , you will hear yes and no. Found this may want to read :
http://badmanstropicalfish.com/articles/article22.htmlRES and salt I used it in baths for treatments only and works. But RES's do not like brackish water ! For decades have not seen much evidence adding salt permanently having any benefits.