It's been a good while since I messed around with WT, but I can give it a shot.
Looks to me like it'd work fine on any number of people due to it being Permanent, but it can only give commands to on one person at a time. So the Worm couldn't screech 'KILL THE INTERLOPERS' at a group of thralls and command all of them. Same for the telepathic link. Also, this is just my take, but making the injection a separate power could be important, since I imagine injecting a literal Man of Steel with a worm could be tricky, and as is no amount of armor would protect against it. Also, as is, the worm can control robots, sentient light beings, etc., so you may want to give it a flaw so it only works on organics.