If I purchase a homekit ceiling fan and connect it to a homekit wall switch, will they interfere with each other?
you do not want to do that.
if the fan is "smart" then it needs constant power, and it will handle turning itself off and on.
If you put a homekit wall switch on the power going to the fan, then if the wall switch is off, and you send a command to the fan, nothing will happen, since the electronics in the fan are powered off and can't do anything. And even worse would be to put a dimmer and send the fan partial power, you could potentially damage the fan that way.
You can install the switch and "hot wire" around it, so the fan always has power, and there's nothing connected to the load side of the switch, and then make automations that turn the fan on/off when the switch turns on/off.
Or you can get a homekit button and stick it to the wall close to the switch. Most buttons support short, long, and double press. so you could assign Hi, low, off to those.