I’ve previously recommended, what in my opinion is, minimally invasive way to disable the 900 Mhz radio, a blob of solder is all you need:
As far as power, if you have ~24Vac ~1A supply from your old controller, feel free to use that, just be sure not to use 24V + and - terminals on your controller as power input, you’ll blow out the filter inductor if you do, as these terminals are designed for low power output only.
You’d need to cut and spice the terminal from the Rachio power brick with the old supply.
Rachio is driver limited to channels 1 through 11 for 2.4 Ghz Wifi, if you are running into major issues during setup, check to make sure you are not using 12+ on your access point.