Yes, that is how you use it. The only problem with your solution is that many things don't update properly in EVENT_SPAWN, due to it being handled at the same time the NPC is actually being sent to the client. So, to make it work properly 100% of the time, you can just start a timer from event spawn and set it to something like 1 second. Then, the NPC will spawn and a second later it will change its features randomly. Don't forget to stop the timer in the EVENT_TIMER or you will be changing the random features every second.
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