That's common and it's a dominance display. I even had pullets mount other pullets!
WHO... told you they can vent check a chicken at (was it) 10-12 weeks old? Do they work for a hatchery? Are they avian trained veterinarians? I think they are BS-er's! :confused: BUT... if they have a technique...
Looks to be a cockerel. :)
Tail feather are irrelevant, or often tricky. It's saddle feathers that count and that's only if they're coming in already... Often starting around 12 weeks but some much later.
A natural standing side shot is the best to give clues. :pop