Sorry you are having issues.
"The 9.6% Corid liquid dose I use is 10ml (2 teaspoons) per gallon for 5 days, then 2.5ml (1/2 teaspoon) per gallon for 7 days. Make fresh daily.
The 20% Corid powder dose I use is 1.5 teaspoons per gallon for 5 days, then 1/2 teaspoon for 7 days. Make fresh daily.
Medicated water must be their only source of water (no creeks, puddles ponds, etc.)"
Don't delay treatment, start as soon as possible. You can also make a drench with the corid to give to an individual bird if it's very sick.
If you have liquid corid, just use a drop or two straight (the dose using the 9.6% liquid is 0.02 ml per 100 grams bird weight (one drop = ~0.02 ml). For the powder you mix 1/2 tsp of powder in 10 ml of water. Dose the bird 0.07 ml of that mixture per 100 grams of weight of the bird, once a day for up to three days. The drench is used in addition to the treatment above, if neccessary.
After treatment follow up with vitamins and probiotics in the water for a few days. No vitamins until treatment is complete.
The medicated feed can help prevent an outbreak, but is low dose, once an outbreak occurs you have to treat. Also, there is a coccidiosis vaccine, and if the chicks were vaccinated, then the medicated feed nullifies the vaccine.
I don't have ducks so cannot speak from experience, but searching came up with this link....
http://www.shagbarkbantams.com/page7.htm
Near the end says that ducks can be treated with corid. I have also read the opposite, so you may want to research that more. Over all corid is pretty safe. It's not an antibiotic, it's a thiamine blocker.
Sorry I'm not better informed on ducks/geese. You could also post that question in the 'ducks' forum here.
The silkies should be fine with Corid. If it were me, I'd probably treat all, rather than take a chance, but that's me. Otherwise, I'd keep them very separate until treatment is over and all is well, and keep a close eye for any signs of illness if you don't treat all. Since you've never had cocci issues before, it's possible that one of the chicks brought it in. When you integrate them into your main flock you should watch for signs of illness in your other birds also, in case they've never been exposed to whatever strain it is.
Make sure your brooder is as clean as you can keep it and no wet or damp areas. That is the best prevention.
Best of luck.