Pretty much all your answers are here: http://www.ex-astris-scientia.org/articles/nebula.htm
There are arguably more than the two pod designs seen on the Phoenix and then the Sutherland et.al. - Sisko may have served on a variant with two smaller warp nacelles instead of a pod, since he has a model of it on his desk on DS9.
Can they still be called Nebula class despite having different shapes and sizes of saucer, secondary hull and pod? Probably... There are eight (soon to be 11!) principal variants of the Boeing 737 that fall into two general "looks" of the aircraft, not counting different lengths of the main hull. They're all 737s, though. OTOH, there are quite a few variants of the Arleigh Burke class destroyer (including those operated by other countries) and they all have different class names..