Choo choo wally, I have been using dcs and legacy together since legacy came out. No issues what so ever. Just get the legacy 990 base set up and plug in the wall wart. Then hook up run the one signal wire to the outside rail of the track ( any 16 gauge wire will do.) Then purchase the new legacy/tmcc db9 cable($20 part I belive)from mth and hook it up as directed.
If you need to hook other boxes or a pc to the db9 serial port. Lionel supplies a db9 wye cable in the legacy set. Just hook it up per the Lionel instructions. The cable hooks into the legacy base then one end hooks to the mth cable and the other end to another device such as Lionel control boxes or a pc. I hooked my old TMCC command base up to there so as to be able to have a third remote .
I can control all of my trains from the DCS remote. The legacy engines have things like the quilling whistle that DCS can't control. So when I run my engines I use there respective contolers with them.Legacy remote with legacy engines. Dcs remote with ps2 engines.
I now run more legacy engines than ps2. The legacy remote to me is easier to use and more fun. I also like the legacy sounds and slow speed control over ps2 engines.
This may sound complicated reading it here, but it is all very simple. As gunnrunner has stated you now have the ability to run every DCS, TMCC, Legacy and convetional engine out there.
Hope this helps.