Actually, Wheel weights are pretty hard, and brittle, as they have both tin and some antimony in the mix. I suspect your cast balls will be smaller in diameter than those made of pure lead, and they will be lighter. How they will shoot will depend on how much less in diameter they are compared to your standard lead balls, and the patch and ball combination you reach for those hard balls.
No, they won't hurt the gun, as the cloth patch will keep the lead from touching the bore of a smoothie, or rifle. However, You will need to weight them, and sort them by weight, if you want to do any accurate shooting at small targets. Depending on what variations you get in the diameters of these balls, you may have to sort the balls you use by diameter, too.
The reason to stick with pure lead is to avoid all these problems. Its one thing to cast a hard ball for a specific hunt, but quite another to cast them up just because!
Pee on the electric fence if you really insist....