Is there a rule on these boards that we can't post suggesting another more appropriate board?
Whether we allow an outside link or not depends on the context of the thread here and what information is available on the outside link, as well as any relationship the person posting the link has with the linked site. Links to other sites aren't banned, but we do look at them with a critical eye.
If someone's question is something we'd consider on topic here, just suggesting they ask on a different site instead is generally going to be removed. If it's not on topic here, we'll most likely remove the whole thread anyway.