I have to agree - I've had one in my 130litre RSM for nearly a year and she is fat, healthy, and happy - and I can still find pods running around on the glass. I think there is a lot more to a healthy tank than water volume and rock weight - things like the nature of the rock / surface area, feeding regimes etc., and the other stock in the tank all have an impact.
That said, the more cautious approach advocated by others is worth considering - I would not recommend putting a Mandarin into a new tank, especially if you are a marine newby. There are lots of things you come to learn about marine through experience, and that experience puts you in a far better position to be able to keep Mandarins. They aren't the sort of fish that you should just chuck in the tank and hope. They are an awesome little fish - which is why people should come to understand as much about all the factors that make for a healthy environment BEFORE they take resposibility for keeping one.