Before I address your concern, there's something I myself is concerned about.
It's the cage location. I do remember you saying that the cage goes outside, but is there at least a roof over their heads? Even with clipped wings, cockatiels can fly surprisingly well if their wings are strong enough. In fact, Lucy's wings were clipped when she flew away. Since Coco also came from Carolyn, I would expect him to be a strong flyer, as well.
Anyways, in my experience, I found that male cockatiels will sing and heart wing (which is what they do when they open their wings like that) to just about anything. This will include birds of the same gender. I had two males, and one would sing to his brother all the time. And then I had a male that would sing and heartwing to nothing in particular. It's not really a behavior that's exclusive to mating. It's just something male cockatiels do basically to show off, even when there is no mate available.
But if you are concerned about breeding, then you could just try and keep Coco separated from them until you figure out who's the female. I remember you saying before that he's usually out all day and has a small cage for sleeping in, correct? So you might already be ok.
In the video, Kiwi doesn't look interested. I would only be concerned if either pip or kiwi showed body language that meant they are ok with Coco mounting them.
I'll use eddieparrot5's video as an example (hope you don't mind, Eddie).
Hey look, the [video][/video] thing works for photobucket too!
Anyways, while I don't usually see them walk side to side like that, female cockatiels will usually hold that pose when they want to mate.