Rather than purchasing two stone molds I would just get the 20" stone mold. If you are not strong enough to lift a stone in the 280-300 lbs range then you could always find someone who is local who can make you an 18" stone or you can train with other implements (keg, sandbag) to work your way up.
You can train with less than (4) stones but if you wanted to make a series and had a 20" mold then I would make a 290, 320-330, 350-360, and 375-390.
A 120 lbs stone would be useless for a man who competes in strongman unless you plan to throw it. Even for a female competitor it would be lighter than what they would typically see in competition.
Movements that have good carryover to stone lifting would include Stiff Leg Deadlift (picking the stone), Front Squat (standing up with the stone), Hang Cleans or Tire Flip (triple extension), and any other type of loading movements such as keg or sandbag loads.
*I would recommend purchasing a couple of different types of tacky and also try taping your forearms with duct tape or athletic tape - especially if you live in warm climates. If you do not want to deal with tape you can pick up Spartan Sleeves which are leather arm guards, they will save you money on tacky and also provide better grip when loading stones.