Can you give some more info about your plan? There are some diff ways to treat diseases so it is hard to say without knowing what you're trying to do.
Assuming that you are removing all the fish from the DT into a 55g hospital tank, and leaving the DT fallow for 72 days to rid it of ich, I think the 55 will get crowded. Squeezing fish together for a few days for TTM is diff from housing them for months, cause they will have a chance to establish territory and fight over it. But I'm not sure how big a tank you would even need to house 7 clowns properly, it's not common to see so many together.