I suppose that ideally you should set up two brooders if you are worried about this issue, and also ducklings are so much messier than chicks that it might be easier to raise them apart from each other. I don't think there is any problem about giving chicks unmedicated crumb so long as you can keep their brooder as dry and clean as possible, especially around the drinker, but this is impossible if ducklings are in there with the chicks. The reason for using medicated crumb is to prevent coccidiosis, which is more likely to start up if the oocysts breed in damp warm dirty conditions. The only time I raised ducklings I used unmedicated crumb and they were fine. The medication is precautionary, and personally I've not seen anywhere that says its OK for ducklings to eat it. There doesn't seem any point in risking it, does there?