To my knowledge the 54" and 60 " buckets for the b3200 both work with the quick attach. The QA is installed between the loader arms and bucket. There are pictures of it somewhere if you do a search you might find it. I have it and the 60 inch bucket for my b2620 (it fits all the newer b's loaders). The QA for the new B loaders (364,504 etc) will not fit a B7800 and probably not B2630 or B3030.
Kubota is saying saying the 60 inch bucket is for light materials I'm guessing because of the lighter metal (compared to a commercial machine's heavy gauge bucket) and the span of it but it seems to handle most residential uses fine. It's the same gauge metal as my 50 inch bucket and probably the 54 inch as well. You can pick up rock and gravel, you just might not want to overload it. Especially with an 800 pound rock or something. A toothbar should work fine on it, doesn't come anywhere near the QA.