Fire buttons A becomes Fire2 on Amstrad, and Select button becomes Fire 1 on Amstrad.
Not always practical, yet usable. Also if the game allows to re-define joystick/controls (not only keyboard) with enough flexibility, and if it can stand with the 2nd button, then it is 100% compatible...really, just "redefine keys" each time you run the programm.
Also the 16bit SEGA paddle is a lot better than the poor and faulty Amstrad Pad, so it remains a nice choice for every gamers, often and quite often easy to find.
You may even also get your hands on the rare "Amstrad Megadrive paddle " thx to the Amstrad PC with the megadrive inside(Amstrad Mega-PC ?), a must -have to all of us...
Those SMS standard controlers may be used alsmot unchanged on a CPC if the software allows to redefine controls. As a result you may use only one button : the fire 2 wich would be the button 1 on the controller.
The use of 2 buttons would need a "re-wiring" solution (a "flexible" modular adaptator may be fine) so it can't always fit unchanged on Plus range.
It seems the "Power" button on SMS controlers is used when the controler is fitted with AutoFire option (needs to be checked) or when using the Lightgun.