M7's mixed with earlier M Units?

I am curious if the M7 units are being mixed with earlier units or being used as solely solidly M7 consists.

I've never seen that done, all trains that use M7 cars have been all M7's. I don't know if its possible to mix them, but I wouldnt see them doing that anyways right now.

It is IMPOSSIBLE to mix & match m1s & 3s with 7s. Completely different configurations. All M7 trains have to be ONLY M7.

Correct they can only be towed by locomotives with two consist together, on the M7's are two compromise couplers, one Locomotive /M7 the other M1/3/M7. they can not MU together, different brake system and couplers all together.

M-7s are AC traction, M-1/3s are DC.

Traction has nothing to do with it, if controls were same they would work together a AC locomotive can be hooked up to standard DC locomotive due to 27 point jumper being same.

Lets not get into the same habit as NJ transit. Transit still does not get the big picture. M7s are not compatible with the older cars.