Should we allow pinging users who can't talk in chat?
It is kind of pointless, specially if the user you pinged isn't going to reach the chat threshold anywhere soon. But as users can join the chatroom while they can't talk yet I don't see why we would need to limit the features any further.
Since a user with less rep cannot reply to the ping, is it OK to notify them?
It depends. If the ping is aimed to be welcoming, inclusive and encouraging to have users reach the chat privilege I see no problem. If the @-mention is used to ridicule such users it definitely isn't and against the be nice policy. It is even more rude and disheartening due to the fact that the user can't respond directly. Such chat messages are subject to be flagged.
Do note that users that are chat banned can also join a chatroom and be pinged without being able to talk.