Going back to the OP's original question of "Are c-clamps safe for mounting movers" I think you have to define what you're calling safe. Will two C-clamps keep the relatively small moving
fixture in question up in the air? More than likely. Is that a good definition of safe, not really. As previously stated it's hard to really tell what a
C-clamp is rated for ("standard for lighting fixtures under 50lbs." is the best I could find), half couplers (a.k.a. Cheseburrows) also are common for hanging moving lights but I can't find any with a listed
rating right now. Mega Claws usually have a listed
rating (500 kg's is the first I found). Once you know what your attachment hardware is rated for you can actually be sure that it is safe to hang your light with this hardware.
500kg rated clamps for a 20 kg light might seem like over kill, but it's a good precedence to set so no one comes in down the
road and tries to hang some really heavy moving light off two C-clamps.
In the end if you are asking this because you don't have the budget to buy rated hardware, but you still want to use the lights I would probably keep using the C-clamps while trying to get the money for the right stuff. If you're asking because you want to know what is safe and correct, buy the Mega Claws or something similar. They are rated for the application and becoming more and more of a standard for moving lights.
As an aside I don't recommend the Omega clamps. Go with Mega Claws.