Programming the DX7s is very similar to the DX8. I just recently bought a used Spektrum DX8, so I’m not an expert on programming the DX and haven't even tried to program it for my D2500.
What I've noticed is, Spektrum doesn’t assign channel numbers, but instead gives them acronyms which represent functionality. The channels get reassigned based on the Wing configuration menu settings. Unfortunately, the labels on the Rx don’t change accordingly.
You can use the DX’s Monitor function located in the Setup Menu to see how the channels are reassigned.
From the DX7s user manual…
And again from the user manual, here’s the Wing Type setup menu options.
Wing types “Dual Ail” and “Flaperon” have the same channel assignments. Choosing “Flaperons” simply enables the Flap System programable features. So, there may be a reason for this awkward enable feature, but you need to change the wing type to enable flaperons for using ailerons as flaps (less desirable stability on D2500…. i.e., wings with no separate flap servos).
With the AR500 Rx, you only have 5 channels, so you can’t set up flaperons with four wing servos (6 channels Rx required). As BG already pointed out, one flap servo on the D2500 is mounted backwards, so with a 6-channel Rx, for flaperons, you’d need to be able to program one flap servo to reverse. Not sure if DX-7s supports this kind of reverse, if not, then you’d need to do it by taking the servo apart and reverse motor wires it inside.
So with the AR500 and the D2500 flap option installed, you’d need to install a ‘Y’ cable using one channel for flaps.
There’s a nice worksheet in the DX7s user manual that might help to see what functions are programmable. Nice worksheet, but I don’t understand the ELEV-RUDD Mix block next to D/R Expo block, must be a typo there.
When I get mine programmed, I'll post a completed worksheet. That will probably be later than sooner. Hope this helps.