Pokemon you receive/interact to battle with can indeed count as the first Pokemon of a given area. So yeah, both Lapras and Snorlax are viable for a Nuzlocke Single... Though, not particularly good, since you can't get 'em until very late in the challenge; especially Lapras.
Hm... Perhaps I should allow trading in Pokes for Gen 1 games, even if they're not eggs. Otherwise, we'll never see challenges with Pokes like Lapras or Pinsir. Not that many people have two RBY games, two GBs, and a trade cable though.
