Cpl or polarisers, there are two versions, make sure you buy the right one, ( again the more expensive one ) is the one for digital cameras, the other won't work as its a none digital version.
A CPL is a "
Circular-
Po
Lariser", as opposed to a normal, or linear polariser, yes, there are two types of polariser, but only one type of CPL... at least as far as the polariser goes.
Possible to use a linear poilariser on a Digital camera, or a circular polariser on a manual focus film camera. they aren't exclusive.
The CPL isn't a 'digital' polariser, or a polariser for digital cameras; it came into popular use in the '90's with the advent of Auto-Focus, where liniar polarisers could effect the operation of the AF system; hence carried over into the Digital-Era where most digital SLR's are AF.
In either instance, it's worth noting that the filter is attached to the front of the lens, and the polarising effect changes with the rotation of the filter; if AF lenses rotate the front element as they focus the focus can change the polariser orientation hence effect, as you press the shutter-release, or adjusting polariser orientation, you may adjust focus.
I assume I need to buy for my biggest lens and then get step down rings ?
That would be the more sensible advice, yes. And for any filter, not just a polariser.
Using step-up rings, you risk the smaller filter-ring vignetting, or masking, the corners and edges of the frame.
But this is not confined to 'just' the filter-thread diameter; if you 'stack' filters then the distance away from the front element can also result in them encroaching into the frame area.
'Slot-System' filter holders can also do this on some lenses, and or if stacked with screw-fits, to push the front ahead of the front lens element. Take heed if you have a UV or skylight on the front as a 'protection' filter before you begin, adding more.
Pay heed also. if using step-down rings, particularly if using a number of them to step-down each thread size; each step-down ring effectively adds a filter thickness to the protrusion in-front of the front element; if you have to step-down from say 77mm thread diameter to 49mm thread diameter, that could be a lot of step-down rings.
The ideal is to pick a filter for the size of lens you expect to most use it on. Given modern vogue for zoom lenses, it is not like of old, when you might have wanted to use one filter, on, well using my old M42 'Prime' outfit as example, 29mm, 50mm, 135mm & 300mm, and quite possibly two or three in between. On my Digital Outfit, I have two CPL's one for the 18-55 and one for the 55-300. Saves any faff with step-=down rings, or even swapping the filter twixt lenses. And the filters are, comparatively cheap enough these days, that the small difference between buying a second filter and a set of adapter rings is pretty negligible.