You probably already know all of this, but hopefully if not, this will clarify:
1) The Lionel System, either TMCC or Legacy, can only control Lionel TMCC or Legacy boards and not any MTH boards.
2) TMCC is the older control system consisting of a Cab-1 Controller and a Command Base with cables.
3) Legacy is the newer system with many additional advanced features. Lot of neat options out there (unfortunately, many out of my price range).
4) DCS is MTH's system that operates PS2 and PS3 boards on command. DCS can control Lionel Legacy, Lionel TMCC, and engines with TMCC upgraded (such as the ERR boards)
5) For DCS to do that, you also must have at the minimum the Lionel Cab-1 and the Command Base plus an additional cable to connect to the TIU.
6) Each upgrade (whether upgrading a proto sound 1 to proto sound 2 or upgrading using the ERR boards on non-MTH engines) will cost a couple of hundred dollars each. You can do your own install, but one had best know what they are doing since you can fry a board quickly and be out that money with no warranty).
7) There use to be another company called TAS (Trains America Studios) that had boards available for non-fly wheel engines. I used those to make two post-war engines command ready through TMCC. They are no longer in business and the top person of that company is now in charge of the Lionel repair division, Mike Reagan.
8) My advice, for what is worth, since you already have the DCS system (which does run both command and conventional) is to stick with that and get a hold of GGG here on the boards (you can look his e-mail address on the search option here) and see if he has any boards available for your Protosound 1 engines to upgrade. Play around with that for awhile and then see if it is worth it to buy the other control system and the upgrade boards to do command with those. In the meantime, you might find other PS2 engines on the buy-sell forum here to add to your collection. No matter what, it will be an investment.
Hope this helps.
Rick