Easy answer first. Fitted with (SH-51) cleats, they'll work fine with M424s. The cage won't obstruct. I have both.

As to sizing, I have three pairs of Northwave shoes including a pair of these. In each instance they run true to actual foot size with the caveat that Northwave shoes are built on a wider last than Shimano which run narrower than standard IMO&E.

For instance, I'm an EU41 or UK 7½, but with a wider foot and high instep. I'd never fit into even a Shimano 44 with heel float and toe overhang like clown shoes because of their narrowness, yet a Northwave 41 fits me almost as if they were hand made for me.

So if you know what your actual foot/shoe size is, buy that size in Northwave.

with craft or stanley knife. then tear it of the main sole. Once it's off you would have to glue it back down. Not something that can be done repeatedly.