I didn’t know that you can “Turn of” the normal image sizes. But know, you can’t add a custom size with the same name as a built in size any more than you can remove it.
And to be honest, I’m not 100% sure that the custom image sizes can be removed in the filter I mentioned above, because I’ve never tried it, but I don’t see why you can’t remove them. Even if you do I think you’re going to need to have those standard sizes.
