Our daughters ten gallon tank has also struggled with ich from the very beginning (for 2 months now).
Malachite green would stress out the fish and kill most.
I found that in general "messing" with the tank too much would lead to other stuff like fin rot and I also found that once fin rot set in on top of severe ich, the fish would die (this of course is not a rule, it just happened to happen in this tank). And with "messing" I mean adding fish while not done with treatment, adding malachite green and salt (that can't be fun for the fish), adding new plants, or taking out stuff. Having guppies have fry (not much we could do about that..) Changing the filter and by doing so throwing away all healthy bacteria in the old filter... all the silly newby stuff that we did.
After asking over and over again, we tried the salt treatment. We slowly raised the temp to 82/83 (I would have liked it higher but the room the tank is kept in is too cold and we don't have an extra heater) and slowly, slowly added aquarium salt (mixed 2 teaspoons to the gallon in a smaller container and added it in over a period of 24 hours). We added extra air by adding a airstone.
Then, as I started to see the fish look clean I started doing 50% water changes every day, adding in the salt in the new water at once (so I took out about 5 gallon and added 10 teaspoons to the new water).
We have managed to kill a bit of the fry (this was the third round of ich, as I did not manage to kill it the first two cycles, so it was rather terrible, some of the fry looked like a little salt bomb) but right now the fish look all very happy and I hope that in a week or so I can slowly take out the salt with water changes.
We have a small pleco in the tank and I does well, even with the salt added.
I think it might not have so much to do with adding new fish to your tank, I think you may have never completely killed all the ich (if the water is rather cool, it might take 2 weeks or more for the ich to start a new cycle) so every time you add a new fish, the fish probably sits on the gravel for a little while to recover from the "new tank shock" and hopla, the ich jumps onboard. If this is the case, quarantening a new fish isn't going to help any, as it isn't the new fish that brings ich in the tank...
L.