Can I use payroll and accounts with one email and login?

Yes you can. Once you have signed up for one of the products, simply navigate your way to the settings tab at the top right of the page, select product settings, and add the product of your choice.

Yes ofcourse..you can customize the settings.

That depends on the Accounts software you are using thou, Many softerware has features of letting you do so such as Sage line50.