Generally hock clicking is normal (or so I've been told). I would talk to a vet, get a flexion test done and if you still have concerns talk to your vet about treatment options.
Adequan is generally more for arthritis or horses with a lot of stress on their cartilage because it helps to build and repair the cartilage. Legend (IV) once a month helps with the joint fluid. And it is not so much the amount of fluid we worry about, but the viscosity. We want very thick joint fluid, thin fluid doesn't cushion as well. If you deem that joint injections are your best bet, I would go for the best brand name Hylarounic Acid, because you want the thickest stuff (In the land of HA $$=viscosity). For the IV injection, you can go with generic because it doesn't matter too much about the viscosity.