Yes, that's it. I think that does what we want.
I think we also need some similar kind of wording for Crush, Frightful Presence, and Tail Sweep. Something like
Crush (Ex): A weredragon can use the crush special attack in hybrid or dragon form if the base dragon has the crush special attack when advanced to the same size category.
Does that work for you?
Rather than "when advanced to the same size category" it needs its current size to be Huge or larger, which is already in the writeup.
Also, it should be "dragon type" not "base dragon" as that'd means only weredragons whose dragon age category was Huge or larger could Crush and we want Wyrmling Giant dragons to be able to make that attack.
So wouldn't the following do:
Crush (Ex): A weredragon can make crush attacks in hybrid or dragon form if its dragon type has the crush special attack.
This special attack allows a flying or jumping weredragon in a dragon form or hybrid form of at least Huge size to land on opponents as a standard action, using its whole body to crush them. …
Tail Sweep (Ex): If a weredragon's dragon type possesses the tail sweep attack, the weredragon can make tail sweeps when in a dragon form or hybrid form that meets the special attack's size restrictions.
As for the Frightful Presence, I'm game for making that based on racial hit dice or total hit dice, since a Weredragon with lots of levels ought to be scarier.
Frightful Presence (Su): A weredragon whose [total racial hit dice/total hit dice?] equal a young adult or older dragon of its dragon type can unsettle foes with its mere presence. The frightful presence is the same as a true dragon advanced to the weredragon's [total racial hit dice/total hit dice?]. For example, a #### weredragon with # dragon hit dice and # [giant hit dice/class hit dice?] will have a breath weapon as powerful as a ## hit dice [age category] ### dragon.
For the example I'd use either a giant weredragon or a weredragon with a lot of class levels in order to hit the Young Adult hit dice threshold.