I've split this discussion to its own topic and moved it to the Level Design board, as I think that's a more appropriate place for it.
In terms of grams88's actual question: I see nothing wrong with this at all, and in fact I would consider it a good idea in the case where you have a pack you overall aren't too proud of but have a few levels you (or the audience) really liked. I've done this myself a few times - most notably, Lemmings Plus I, which contains mostly original levels but has a few that are ported from my (otherwise-mostly-crap) previous packs on Cheapo and WinLemm, some examples including "Return Of The T", "No Time To Die", "Oblivion" and "To The End!".