That's a pretty tank, and the video was really useful in trying to determine what's going on. You have a male and a female who are exhibiting pre-courting behavior, and these little skirmishes are the first steps in formation of a pair bond. The key thing is that these episodes are followed by relative calm, during which the two fishes swim together happily. This is a very good sign. So unless the male becomes relentless in his pursuit of the female, to the point where he may actually damage her, you have nothing to worry about, but do keep an eye on them. Chasing the gourami is just a secondary behavior related to territory defense, which goes hand-in-hand with pair bond formation.
Don't be so sure that they are too young to be exhibiting these behaviours; my Mum thought I was too young as well, when i was 15.
Keyholes are wonderful fishes, and once a pair bond is formed, it tends to be very stable. That said, it's usually best to give the male a few females to choose from initially, but sometimes a young pair will work it out themselves. Fingers crossed that happens here. Good luck!