Don't know about the 7C with servo slow, I have a 10C & it only has servo slow on throttle (to replicate the slow throttle response of a turbine I would assume).
There is a 'mix this, reassign this, modify this fix' but I just gave up & programmed the Hitec digital servos with the Hitec programing box.
Isn't there a device that I can plug in between the servo and receiver?
Yes, I bought one years ago, a 2 channel device that I use on retracts & gear doors & it works perfectly.
Like SCALECRAFT I also have a JR 9 channel & yes, it has servo slow on all channels I think. You can
even have different speeds up & down, nice. - John.