Serperior seems fit to be Pride, i mean his title is Regal after all.
Cubchoo can be Envy, if i were him i'd be envious of others that don't have a cold 24/7.
Snorlax is Greed because he takes all the food for himself and doesn't share.
Slakoth is Sloth for obvious reasons.
Gyarados seems to be quite the angry and Wrathful fellow.
Munchlax or Snorlax can both be Gluttony.
Idk about lust.