When playing the initial 30 points, can I use a tile from an existing meld on the board to complete my meld without using the points from that tile? For instance, I have a red 9, 11, and 12 in my rack, which is 32 points, but I need the red 10 on the board to play them. Is this a legal move?
From the rules (emphasis added):
After players have made their initial plays, they can also build on other sets on the table...
I think this implies that while you are making your initial meld, you cannot use the existing melds.