What exactly do you mean by "Plain Sets"?
Oh sorry, maybe this is due to me being non-native speaker hence often take wrong words or grammer.
Within this format only cards from on sets may be used to assemble a deck (hence my reference to the LoC constructed) - for Lost Souls and Doms this might be adjustable since maybe not all sets provide enough different Lost Souls (e.g. any meek Lost Soul could be added and Dominants from an overall predefined set of Doms - all other cards must only be used from one set).
Then you could establish different categories too: One for matches of decks only made out of cards from the same set (like the LoC constructed) and another category where decks made out of different sets can be played against each other. (Hopefully this makes it a little bit more clear ...)