I'm not very familiar with salt (never use it), but I don't believe it's something that you dose more than once. It stays in the tank, so the only time you would need to add more is after a water change.
6 serpaes in a 6 gallon is not a tank set up for success - sounds like you are aware of that now. As long as you don't make that mistake again they will not have died in vain.
Ich is in the tank, and yes it will die without hosts. However, that doesn't mean that you have to nuke the tank and start all over. I would put another fish in there, only appropriate for the tank size this time, and proceed with the heat treatment. Ich is very easy to take care of, provided it's caught early. I do preemptive heat treatments on all new fish, so it's not something you have to wait till you see spots to do. I have never used salt - the heat alone works - so I would do a massive water change to get it out first. I'm sure those that use salt will say to leave it in, but I am anti-salt so I say get it out. There's always more than one way to do anything, so it's just a matter of what people have found works best for them.
Also, moving forward, the heat treatment is best administered when it's caught early on. If the fish are infested (spots everywhere, clamped fins, moping around the tank), then meds are the way to go as they need immediate relief. Quick cure is my preferred med, though it's been ages since I've had to use it for ich. The heat treatment takes several days - time a dying fish just doesn't have. Quick cure works in 2 days. Is there a chance the meds will kill the fish? yes, but a fish that is that infested is already knocking on deaths door, so it's a little late to be concerned about that. The reward of timely relief outweighs the risk of killing a fish that's near death as it is, in my opinion.