Hmm, saving sick fish is definitely not a good idea. Especially if you already have existing fish, big no no. Definitely follow some strict quarantine guidelines or your other stock will feel the pain.
Since you already started treatment with Esha Hex, continue with that treatment. The amount of time is up to you but if the fish are in bad shape, I'd go 5-7 days. Just my opinion. Make sure you keep the tank spotless, vacumming feces constantly.
Eddie