[Eric Paddon]

It's not that Iblis is the Imperious Leader, but that Iblis in effect gave birth to the Cylon race a thousand yahrens ago by using his powers to bring about the downfall of the reptilian Cylons and the rise of the robotic Cylons. And so his voice was used for all of the Imperious Leaders. That's the key point in the dialogue exchange in Baltar's cell. Iblis is revealed to have been the creator of the Cylons as we know them (he likely used another form when he brought down the original Cylon race).

As to his ability to compel Baltar, some have suggested that it was the SOL that compelled Baltar's surrender and not Iblis. Though that idea was I think in the initial version of WOTG that wasn't factoring in the deeper connection of Iblis to the Cylons, which it has to be acknowledged came about solely because Larson decided to capitalize on the fact that he'd gotten Patrick Macnee for this part. If any other actor had been cast in the role....this vital piece of backstory in the history of the Cylons would never have existed in the series.

If Iblis is in effect the founder of the Cylon race who has perhaps by extension the power to compel the Cylons to obey him (I have always wondered what happened to the Baseship after Baltar surrendered. Where did it go? To me, the answer might lie in Iblis using his own power to "remove" the Baseship from the vicinity lest it interfere with his plans for the Galactica at this point) then perhaps because Baltar has made himself a Cylon during this time, Iblis has some "control" of sorts over him?