Simple? MPX Profi 4000 ?
PPM9 is the standard mode to use for a PPM Rx no matter how many channels the Rx has. PPM7 is provided just to give backwards compatibility with an old Mpx Rx that you are very unlikely to have.
The difference in plugs is that they are a “label” for the timing that is designed into the servo. Mpx servo timing is centre 1.6ms +- 0.55ms. That is what your Tx sends in Mpx mode, and the servos are designed to work on that. Realising that they could sell servos to non-Mpx users, they also make their servos with uni plugs and uni timing of centre 1.5ms +- 0.4ms. So the plug tells you what the servo’s electronics are timed for. Taking an Mpx plug servo and replacing the plug with a uni plug would still mean the servo operated to Mpx rather than uni timing.
Maybe you don't get a choice where you live, I know they don't in the USA, but in Europe we can buy their servos and Rx with either Mpx or uni plugs and have to specify which when we order.
Harry