I actually think I stole that advice from you RBU:>)
If you JUST started the first dose of Cupramine then the extra fish you add would likely be fine going into that...again, unless it is sensitive.
I had a pair of new clowns in my DT (big mistake, went against my old ways, but wanted to get our old running tank looking good)...one died from my lack of swiftness...the other had alittle ich, but was acting and eating fine...I set up my QT...I did a 75 minute Paraguard bath (though some advised against it), rinsed him, then into QT. I added the first dose of Cupramine later that night....next AM he was covered in ich and I was mad that I made it so much worse...later that night the ich was gone! I waited 48 hours to add another half dose (fish was doing fine), then another 48 hours to get to recommended dose. SO I guess it starts working pretty fast.
IF the fish is doing ok and does not have tons of ich, I think slowly work your way up to the proper doseage.
I try to think of it as us getting shots or taking meds...and how large quantities alter our bodies all at once.
Good luck.