Great stuff definitely floats. Depending on the size of the background, I don't think the magnets would work in holding that in place. Even if did, knowing great stuff you would have gaps in-between and things could get behind there causing problems. Someone from this site actually cut a piece of plexiglass the same size as the back of the tank and mounted a background on there. This way he could switch them out as he wanted. I think if you wanted to do something like that, this would be a better alternative. Making underwater ledges sounds good but with nicely planted tank I don't think you would need it. The water kind I think its gray/black and is more expensive. If you are not going to use silicone and/or cover substrate you might want to use this because the regular stuff is ugly! I made a small "shoreline island" out of great stuff which I made from inspiration from slow foot’s setup. This leaves almost every inch of water available to the newts while providing a nice land area for them to use should they want to. I think this would suit you greatly.
As far as how many to a tank, this of course depends on alot of variables. With your newts being on the smaller side and if you have it heavily planted I think you will have no problem fitting a large group (10+). With large quantities, making sure water quality is maintained and everyone gets food can be increasing more difficult but I think it can be done.
Mitch
