there's two options (and I see issues with both):
a) a complete merge: RM 1v1/TG, DM 1v1/TG for all. All new accounts would either start at 1600 or have a choice between 1200/1400/1600 -> frustration for new players trying 1400 or 1600 + abuse by trolls/smurfs (which can be dealt with using the current ladder bans)
b) merging the lobby, but keeping the threefold ladder system -> the total number of ladders in the quick preview would get too complicated (adding 4 additional ladders in MS) + players would still only look out for games being hosted rather than ladders used (same problem as currently in the NPL -> not distinguishing between RM Beginners & Intermediate)
This is just me speaking from the top of my head atm. I can see some merit in using one lobby+ladder & making more use of the initial rating boost we have now, too (similar as in chess, as someone pointed out before).
OK. I think if you used ip addresses to determine whether a player already has an account you could force all players new to Voobly to start at 1200, then players creating smurfs can only start at a rating they have previously reached. Ie. Once you get an account to 1600 then all future smurfs start at 1600.