Gyarados: Not bad. Gyarados doesn't have too many options, sadly.
Alakazam: I'd give him another attacking move, such as Focus Blast. Which move is up to you, and which one is best to replace depends on what item, nature, EVs, etc. you're using. Sweeping with only one type of move won't always get you very far.
Dragonite: Solid. He has more options, but your set looks fine.
Rhyperior: Good. If Rock Polish works for you, then keep it, but if not, replace it with another coverage move, such as Ice Punch, Avalanche (works well with his low speed), Megahorn, etc.
Umbreon: Replace Dark Pulse with Payback. Umbreon has low attack and pathetic special attack, and since you probably aren't investing too much speed in him, Payback works well.
Meganium: In general, not very good. However, yours is good for a Meganium. I suggest adding Leech Seed over either Toxic or Synthesis to differentiate it more from Umbreon. You may even want to replace both Toxic and Synthesis with Leech Seed and Reflect, then give Meganium the item Light Clay.