In order to end round one with four heroes plus two free allies, you need the following:
1) Sword-thain in your opening hand (and Arwen as one of your heroes)
2) Emery in your opening hand
3) Imrahil and two other allies in your discard.
Arwen + Emery gives you four cards in discard, one of which you can choose -- if Eowyn or Cirdan is the third spirit hero, that can go to five cards in discard, two of which are chosen. But it's not likely. Best case (Arwen-Cirdan-Elanor) has about a 17% chance of both Emery and Sword-Thain in the first eight cards, and only around 10% chance in the first six (when you have to decide if you'll mulligan) -- and even if those comes together, you either need Imrahil to be in the first eight or in the next three cards, *plus* two other allies to come out (probably in the Emery discard) who aren't Emery or Imrahil. I'd expect that to come together 1 time in 20, if that.
Most of the time, using Caldara on turn one will give you two heroes and two allies drawn from your first eight cards -- and you can never use Caldara again. Good for a fast start? Sure. Overpowering? Not hardly. Since it's once per game, the temptation would be to stall until you have a Sword-Thain and Imrahil already in play so that you can get your four allies, but until then you're limping along with a 2-wp spirit hero with no special powers and definitely not being aggro.