I have never really had much luck casting less than 1/16 of an ounce on a slip float. If you want to use a small jig, then you will also need a split shot. This keeps the float from banging on the jig knot, and will also save you from chasing the float down if you get hung up and break off, the weight will keep the float from sliding off the end of the line.
If you're vertical fishing, like through the ice, then you can get by with less. Unless there is a trick that I don't know about.