As far as that video, that's overstocking. They do it with Cichlid tanks fairly often, and you can do it with angels. Something like 3 angels in a tank might not work, but 8 or more will, as the angels have no where to establish territories, they would be constantly chasing other angels, etc. However let's just say opinions on whether or not overstocking is a good idea, can vary greatly.
And if you look at how unhealthy some of those angels look, I think you'll know which I think....
For a Koran and Annularis in a 125gal, I think it would be tough. They both are some of the larger angels out there, and they both, by themselves, would eventually outgrow that 125. The Annularis is one of the more 'multi-angel-tank' friendly angels. The Koran is so-so. Yes, it is a good idea to get the new angel significantly larger, or smaller, then the established angel. I wouldn't remove the Koran, but use eggcrate and divide the tank with them on opposite sides obviously. They will get use to not only the sight of other angel, but the smell as well.
I think as long as you have a backup plan where one of the angels could if they don't pan out, and of course realize that these fish will grow, and that 125gal is only temporary, it's worth a shot. I would also say maybe a 'dither' fish, so not only is the Annularis is the new addition, but there's another new fish as well (as long as you have the room).
