I'll recommend something in a bit, hold on...
EDIT: Okay, let's see:
Magnezone: You already have Discharge on it, so i think Zap Cannon is kinda useless (Mostly because of the accuracy). But if you really want to use Zap Cannon, then giving it Lock-On in place of Barrier might be better. It's only going to help this one move, but hey, it's better than missing all the time. If you want a single hitting electric move though, i'd suggest Charge Beam, since it can raise your Special Attack.
Greninja: I agree with Lore, Greninja is much more Special based, so Water Shuriken is bad. It would be better if Greninja got Skill Link, but even then, it would have to compete with Protean. Yeah, protean would be better anyway. But since you're rocking the Protean, i'd suggest you use Shadow Sneak over Water Shuriken. It gives you priority, and the immunities when you transform into a Ghost type are always good.
Machamp: Ame, Machamp is a prime example as to why we need some other TMs. Anyway, the best thing you could give Machamp that isn't a Fighting move is "Facade", if you get in the game corner. And even then, Machamp can't do nothing against Ghost types. I don't know what i can recommend for it. (You could breed it and get Knock Off, but you already wasted a Link Stone, so whatever for now.)
Metagross: I usually go with 4 attacking moves in other pokemon games, but unless you're using Greninja, i don't think it's a good idea. i was going to say "Use agility", but i don't know what i could take out from it's movepool. Well, if it worked until now, no reason to change it right?
Arcanine: I wouldn't use Justified over Flash Fire. A immunity is always better than a resistance. But well, if it worked, why not right? I like Arcanine with ExtremeSpeed, but i guess you can leave it like that for now.
Gallade: Good.
Based on your team, i can say:
Mamoswine would help with coverage. I think it would help your team a lot. My opinion.
Sylveon is good. I don't even care, Sylveon is a force to be reckon with. It gives an awesome coverage for you team also.
Crobat is also very good. I'm pretty sure you know the power of Crobat's Acrobatics in Reborn.
I'd love to recommend Rotom, but we got no other forms of Rotom yet. Oh well.
Good luck o/