If you want to, go for it. Your current team is excellent already, and there's nothing a Hydreigon could add. You won't be getting one until the post game, and Zweilous isn't all that helpful against the League. For efficiency's sake, it's not worth it at all, as you'd be spending way too long leveling it up, and you already have a Dragon type. However, if you really want one, it certainly functions well in the post game -- just don't expect it to yield much of a result until Level 64.
Hydreigon @ anything
Ability: Levitate
- Dragon Pulse
- Dark Pulse / Crunch
- Earth Power / Earthquake
- Flamethrower
Hope I helped!