It would not make sense, in my opinon, for all the Dalish clans to share the same culture and rules as each other since they'd be living in different areas with some isolated and some close to human settlements.
While most of them strive for an understanding of their lost culture and past, they'd likely have different interpretations of what they find and would interact differently with outsiders depending on previous experiences; a clan that has experienced heavily hostility would likely not be terribly welcoming towards humans or the idea of trading with them versus a clan that has had good experiences and would see a benefit rather than the risk outweighing that potential benefit.
Similarly the situation with mages; one clan might see it as a necessary sacrifice for the good of the clan because of a lack of room, resources, insufficent training or to avoid drawing attention from outsiders while another clan would see casting these people out from the clan as unacceptable and even consider it disrespectful towards their past people's legacy. Trading their mages like in Merill's case is a compromise but in some cases, that might not be an option due to their location or because of lacks with other clans.
TL:DR - For all of the Dalish clans to be rid themselves of their magical youths one way or another would be... odd, in my opinion.