Nice to see you here! This is an exchange, so you could certainly write a Millicent/Hagrid story if you signed up and were matched to someone who wanted one of those characters in a het situation. However, everyone who signs up has to be open to writing one of at least five characters, so I can't promise that whoever you were matched to would be the ideal recipient for a work already in progress. In general, exchange writing goes best when works are inspired by the recipient's request.