Is it possible to combine let’s say a zigbee sensor (Xiaomi) connected to the gateway (Xiaomi) with an action like triggering a Z-wave switch in an automation? I am not sure if both platforms can work together?
Yes of course. Home-Assistant is agnostic. As long as it can get a state from one thing and set the state of another, all you have to do is make an automation to do it.
So I have zwave motion sensors and switches controlling Zigbee bulbs.