I'm between a 5.5 and a 5. I ordered the 5 and it fit perfect.
As for the "toe ridge" issue, I don't seem to have it because my toes go across pretty straight. (As in the front of my little toe is not much further back than the front of my big toe) The top of my little toe reaches approximately the bottom of the nail bed of my big toe)
My guess is that if your toes create more of an angled foot, it may be an issue.
They are super comfortable for me though and very cute.

I was wondering if they are fully submerge-able in water.
Since other people seemed to have that question too, I just called up their customer service to ask them.
They said that since this is synthetic leather and not real leather, that I could fully submerge them in water and even wear them in water for a number of hours and they would not fall apart.  Yay! Can't wait to try these out :)