DSMX is the new version of DSM2. Both use spread spektrum 2.4 technology, but the DSM2 locks into 2 channels, the DSMX continuously searches the band for the best signal and hops to it. This is typical of all the high end 2.4 systems (to the best of my knowledge).
DSMX transmitters have the DSM2 system still in them so if you bind it to a DSM2 receiver it will run in the DSM2 mode. When bound to a DSMX receiver it will run in DSMX mode.
The new DSMX receivers are the same way. If a DSMX transmitter is available it will run in DSMX. If your new DSMX receiver is bound to a DSM2 transmitter, they will both run in DSM2.
The orange Rx's are DSM2 compatible only. My DX8 runs great with the orange rx's in DSM2 mode. Once I switch models to my big balsa cub which has a DSMX rx, the radio transmits via DSMX.
Unless made specifically to work with DSM2 such as the orange rx's, you must use Spektrum rx's with Spektrum tx's. Same goes for futaba, airtronics, turnigy, Hitec, frsky etc. The JR radios have two types of systems. The older ones used DSM2 so they are compatible with Spektrum DSM2. Their new system is called DMSS which is no longer compatible with spektrum so you must use JR rx's.
There are modules available to change the type of system you are using. The Frsky module can be used to convert most anything I believe.
The model match is great. It's a setup so that unless you have selected the correct model in the transmitter that matches what was bound to the receiver, it will not allow you to operate the model. This prevents you from taking off with the wrong settings and avoiding a crash.
Hope that helps.