I'd suggest "reverse" quarantining new fish, at the very least as observational only. I know you are managing ich, but the chances you'll be able to do the same with brook, uronema, or velvet are very slim. So, you'd want to make sure the fish you bring in aren't carrying something WORSE than ich.
Once the new fish have had a week or two to settle into QT and start eating, and if no signs of disease have cropped up... spike the tank. Move a piece or two of sacrificial LR or a container of sand from your tank to the QT. Observe some more. This will hopefully give you an idea of how well the new fish will be able to "manage" ich without the stress of possible bullying and food competition from your established fish. Hopefully, the new fish will be able to tolerate and establish some immunity to your resident parasite, and it will cause less of a parasitic population-boom when he hits the display tank and the stress levels rise while he is integrated into the pecking order.
This is just a theoretical approach, I've never tried it personally. But, I thought a lot about what I would do if I had to give up and go ich-management when my QT failed the first time and I was still trying to catch all of my fish for round two. But, I'm happy I didn't settle for management in the end.