Bluetooth control

Bluetooth is not available currently, as far as I've heard, in the MTH app. 

However, any Lionel loco with Bluetooth (or LionChief) can operate on a DCS layout with independent command control using either the Lionel app on a smartdevice or the Universal LionChief remote (which now includes Bluetooth).  You will not need the Legacy or TMCC hardware to use a LionChief or LionChief+ or Legacy loco that has Bluetooth. Hope this helps. Until recently Legacy locos did not have Bluetooth.  That is changing going forward. 

So short answer, your PS2/3 locos controlled with DCS will reasonably happily co-exist with Lionel Bluetooth/LionChief/LionChief+/Legacy with Bluetooth locos in command mode without adding any hardware that doesn't come with the loco (the LionChief remote comes with each loco, although this will change with LionChief 2.0+).

AFAIK, you will not be able to run the LC BT engine via the MTH App. It requires a separate download of the Lionel BT app to your device. Or be able to add the Lionel LC to the MTH roster w/o additional equipment.

As stated, the two will co-exist on the same layout but not through the same control system.

So, I guess my main concern is if I can run both simultaneously on my trackage. 

Seems I can. 

Other than having to juggle between remotes or apps, it shouldn't be a problem.