I am constantly frustrated with blanket statements in this hobby. I'm a technical guy and I'm in to a lot of different, very technical hobbies and one thing about tank keeping that is different from everything else is that tanks have an element called life. There are certainly best practices that all keepers should be aware of, but this "very large tank required for mandarins" is just flat out wrong.
Mandarins require a very mature/large tank... if they aren't eating prepared/frozen foods.
The only advice I see regularly that is always going to be true is that a wrasse will out-compete a mandarin for food.
I'm sorry guys, but I've seen far too many big, fat, ultra-healthy mandarins in 50 and 60 gallon tanks. These blanket statements are wrong and should be prefaced with, "unless..." but they never are. I see this mandarin thing as not much different from tang police-nonsense.
The best possible advice to give anyone asking about or wanting to add a mandarin is this:
Be prepared to spend time caring for this guy. This is easily the absolute most important of all (other than competition for food). So long as you're willing to devote the time needed, a mandarin can be cared for in just about any environment.
But make no mistake - devotion is absolutely necessary. You have to be willing to feed this guy every single day, if you don't have a fully mature tank with a large pod population. My advice would be to familiarize yourself with what a healthy mandarin looks like and be prepared to move him to a more suitable environment if his health appears to decline. There are plenty of mandarins out there eating prepared foods (mine is one) - why not encourage anyone asking to seek out these mandarins instead of, or in addition to, your blanket statements?
You are dealing with life - and while that should elicit more care on your part, it definitely means anything is possible.