Do you think my Registration class looks logical and fits within your suggestions above?
On the actual topic, you should note that a lot of programming is personal preference. You will easily discover which methods work best for you; if you find that a "God class" works best for you (which I highly discourage), then go ahead and use it.
A general rule for me is that any class should have at least two methods in it. And no class should have so many methods that you're constantly looking for a method within it.
If you think about an Email Validation, it really shouldn't need its own class. It would have only one method. (isValidEmail() or similar.)