Never done a dynasty league before, but I'd done some small keepers before. I think the only fair way to do it is to redraft the whole league with the new players and start fresh. If you all really want the new people in, that's the fairest way to do it. You could use either this season's standing or overall historical standing to determine draft order (or just randomize it).
Otherwise, you're left with a scenario where you force teams to drop some players, and inevitably that will still leave nothing but scraps for the new teams. I suppose you could reduce the bench spots by a couple forcing all teams to make some cuts, and then the two new teams would get first dibs on all the free agents after the cuts. I would think you'd want to give the new team first crack at the rookies in the draft as well. The downside is that it ruins "parity" in the league, because these new teams are going to suck for several years until they can refill their team with rookies. But, this is how the real NFL works too, so I guess that's an option. I know I wouldn't want to join a dynasty league if all I got was everyone's left over free agent junk and a bunch of rookies.
So, since this isn't the real NFL, I think the best option is to dump all the teams and start over with a re-draft from scratch. It puts everyone on the same playing field. Either that, or just keep the 14 teams, and create a waiting list for those 2 new players. They could jump into the league if and when one of the other 14 team owners decides to drop out. It could take several years, but there ya go. I can't really think of any other options that make sense beyond those.