I would advise on not getting a Mandarin until your tank is at least a year old. Mandarin's are very hard to keep mainly because they are quite picky eaters. They are beautiful fish, but I can't understand why they're even sold since the majority of them do not survive in our fish tanks. They feed off copepods and other pods that live on the live rock. An established tank (at least a year+) can provide these pods.
A refugium would be a great idea if you decide to make it a reef tank. A refugium can provide the necessary breeding grounds for pods and, along with caluerpa, can help minimize nitrate levels.