If you are using shoe-fitting fins, but still need to keep your feet warm, a neoprene sock or bootee in 3-5mm will serve you well.
However, if you try to walk around in such socks they will be worn out in no time. Even if they have a textured sole, they are just not built for anything but finning.
With adjustable fins you need a properly designed diving boot. As with your wetsuit a close fit is essential, both to keep you warm and to stop you sliding around when walking.
This can make them a little difficult to pull on, but if this worries you, choose boots with entry zips. Most of those in the guide have them, and although a zip is another entry-point for water, it does allow a better fit.
The soles should look less like trainers than boat shoes. This will make you sure-footed on slippery decks.
Check that the sole is firmly vulcanised to the neoprene upper, and look out for abrasive internal seams in tender places, such as the instep or the back of the ankle.