Trickle vents in old windows?

Can I retro install trickle vents in any windows?

Is it as simple as removing the window from the casement drilling some holes and screwing in the vent?

My only concern is if there are some must avoid windows so I don’t accidentally shatter an insert or anything like that

It is indeed possible . My suggestion would be ... leave the window closed, and with a pencil , mark the the sash where it close against the frame , just marks the upper most point before you ' foul ' the window frame. remove the sash , remove the glass ( saves breaking it! ), now mark where yo want the trickle vent. No need to ' route ' it out like the manufactures do , just lots of holes will be ok , just make sure your drill holes aren't visible outside of the trickle vent! Beware some frames may have metal in them , or may not .

To be honest why not just leave the window open on night vent?

Don't go and ruin your window, and allow uncontrolled noise in! Put it on night vent when you need air, and shut it when you don't.