My GF and I took the Stardust cruise this past January aboard the Costa Fortuna and loved it. I think they had about 10 dance hosts. I think the pricing for Stardust is lower than most of the other dance cruises. The nice thing about a large group are the number of dance venues available.
Unfortunately, the Fortuna is not available next January. I think they are trying for the MSC Poesia, but the details are not out yet.
I have seen some dance cruises scheduled for the Norwegian Epic (which looks incredible in the PR shots), but it has yet to sail, so there is no telling what the actual experience will be like.
Yes. We did one trip on the NCL "Star" a few years ago. We were part of a small group affiliated with USA Dance and had two to four hours of daily instruction for five out of the 10 days. (Too much!) The largest floor was (is) on an upper deck where there is a lot of pitching and rolling when at sea. This floor was not too big and it was often sticky. We thought the ship, food, service and entertainment were quite good. We very much liked the informal dress code (NCL calls it "free-style" cruising), in contrast to the Cunard ships [see below].
We have also sailed on a Costa ship (Mediterranea). We signed up to go with a dance band. (There was no instructional component.) The band was OK, but it was more of a concert band and didn't really understand how to support the dancers [topic for another thread]. The dance floors on this ship were very small by our standards, and the management didn't do a very good job of keeping the public out of our group's venues, so the floors often were more crowded than necessary. The ship was (is) fancy (lots of chrome and mirrors) and we didn't like that there was a lot of smoking allowed. If traveling with a band, make sure you know the band and how they treat dancers before you sign up.