There was no need for the army of geth. But even if there was, they already apparently knew how to get geth passed security scans - as evidenced by them not spotting the geth that Benezia smuggled in on Noveria.
Even if that wouldn't have worked, they could have just shown up with Sovereign and a crapload of geth ships, like they did at the end, and have the geth drop off a bunch of units. With the element of surprise, they wouldn't have needed to appear at the tower.
But the most telling point is that they didn't know what the conduit was. None of the beacons contained that information. So there was no way for them to know that there even was a backdoor on to the citadel.