Small feet and extra shims here. I have Zeros. I do have to be quite precise about the cleat placement on my left shoe (the one with about 1/4" of shims) or it will be impossible to clip in.
I also have a small foot - I wear a Specialized size 36. With small feet you have to be *very* careful to follow the directions about cleat installation and put the correct cleat shims (included) in. If the cleats don't sit level on your shoes you will either not be able to clip in or you will not be able to stay clipped in.
I have found they need a bit of break in, but after that they are not difficult for me to clip in and out from (and I'm light). Definitely get the cleat covers.