I just got an alert to this thread..
No problem running Legacy or TMCC with 2 rail. Hook the legacy (or TMCC) base to the ground rail as you normally would. You will need a Cab-1L and Base -1L to run the Legacy and TMCC locos. A regular Cab1 if you are just running TMCC. With the Cab1L you will have all the needed functions (horn, bell, lights, speed control, smoke, crew talk, etc.) If you want all the features you will have to get the full #990 Legacy System.
No problem running either Legacy or TMCC on the same layout and the same time as DCS. I have done both. I do not go through the DCS TIU for the hookup, but you can. I just prefer to use the Lionel Cab for Lionel locos and DCS remote for DCS locos.
I have no experience running ERR, but I imagine its the same as the above
I have no experience running TMCC/Legacy with DCC. I would guess there are compatibility issues