The Zinc weights melt at significantly higher temp than lead or lead alloy. I don't worry too much about them because they will simpy float on top of the molten lead to be skimmed off. As far as stick on weights, I have noticed a huge disparity in the hardness of these weights. The ones that are thin and segmented seem to be the purest lead. I have found others that are nearly as hard as clip on weights. I segregate and melt them seperately. The fluxed ingots get marked with a sharpie so I can make pretty repeatable alloys.
Typical muzzle loader bullets are 20-1 alloy of lead to tin. Tin can be obtained at plumbing supply stores in the form of 1lb 50/50 solder bars. Two bars and 19 lbs of virgin lead is perfect.