The charge rate is determined by the phone; when you plug in a device the device “negotiates” the voltage and current it wants. A 30 watt charger will not be faster than a 20 watt, but there is no harm in using it. You could also use a 61 watt MacBook charger, but again, it would not be any faster. And you can use the 30 watt for any other Apple devices such as your Apple watch, AirPods, etc. One thing to note; the MagSafe wireless charger is limited to 15 watts, but plugged in it can charge at 20 watts. So using a cable will be faster than wireless charging.
All Apple power sources and cables are cross-compatible as long as the connector fits without forcing. Obviously, a lower power source will be slower, but it will still work.