Can you please describe the "sacs"? Ick is a parasite that needs a living host to survive. It can fall down to the gravel, but will not adhere to glass, rocks, plants, etc. What it sounds like is snail egg sacs... did you put any of the LFS's water into your tank with the fish?
If the danios are not showing signs of ick, I wouldn't worry about them too much just yet.
Quick Cure will surely wipe out your shrimp due to the malachite green in it's contents. The safer medication to use would be Formalin, dose according to directions. I must warn that most medications are not safe for shrimp or other inverts. Quick Cure is a combination of formaldahyde and malachite green, Formalin is the formaldahyde without the malachite green, and tends to work very well.
What kind of catfish are they? Formalin is safe for most fish species, but depending on the species of catfish in your tank, you may need to use it at 1/2 strength to be safe. If I know the species of the catfish, I can let you know which is the safe way to go.
Please take a look at this thread, the truth about ick! http://www.fishforum.com/viewtopic.php?t=1648