Le scraf basically answered your question but I'll answer the part in which you need to replace a Pokemon.
2 dragon Pokemon is fine, I have a whole team of dark types! xD But here are some pointers...
I recommend I fire type because as far as my knowledge goes you don't have one.
(They are sooo needed because White 2 is a struggle without. Found out myself)
You have just one mere defensive Pokemon, add Umbreon or Mandibuzz.
(Sorry, I like dark types)
Although your Dragons have resistance to grass, a lot don't
(And if this is XY then Dragons won't have a lot of resistances.)
Add a fire type. I'd maybe get rid of Dragonite or Metagross since Milotic can defend against fire attacks. Heracross maybe be a bug but his attack is good and you need him.
Hope this helped you decide! :)