Jacki, regarding use of slag, I guess it depends on 1) how fine it is (if it's too fine it wouldn't work as well) and 2) what the slag is made of and if it's mixed with anything else - just make sure it isn't a metal that reacts with cement (see below).
Lynn Olson, longtime concrete artist, says in his book, Sculpting with Cement, that steel (esp galvanized) is extremely stable in concrete as long as it is in complete contact with it, but steel that is coated with another metal can corrode. Copper, bronze & brass may retard setting where they contact the concrete, but they are stable in it. Nickel, chromium, silver & tin resist corrosion. But aluminum corrodes in cement.
Pinch pots would be adorable with your seedlings!
