I have done exactly what you are proposing with a DX7 and AR7000. There are a number of ways it can be done, however the way I did it was like this:-

Elevator servos on the elevator and Aux 2 channels. I used two channels so that I could use the separate adjustments to make sure the two elevator halves were aligned. You need to use a mix with this, use mix 5 or 6 because with these the trim will work properly (ie move both servos).

Aileron servos on the aileron channel using a simple Y lead. In this case the servos need to be arranged mechanically to move in separate directions

Throttle, rudder and gear on the normal channels.

The flaps went on to Aux 1. Here I had a problem because I needed to reverse one servo (couldn't be done mechanically - it was an ARF) and wanted the same separate adjust as for the elevator. The answere here (at least for me) was a JR Multibox (sometimes called a Matchbox). This plugs into a Rx channel and provides up to 4 servo outputs with individual adjustment.

As mentioned above, my main reason for not using Y leads all round was to be able to get indivual fine adjustment of the elevator and flaps.

As ppkk says the AR9000 will not be any benefit as the DX7 will not let you access the extra channels, even using mixes. Would be a nice idea though!

Regards

Terry