Your UIX designer is very likely going to want to brand your app and change the design of the log-in views. All you need to do is tell Devise to generate the views so that you can modify them.
rails g devise:views
If your Dir of Product prefers your users to log-in using a username instead of an e-mail address, you can easily make that change by modifying the User model. Here's how to replace e-mail with username.
rails g migration add_username_to_users username:string
Now update your database.
Then modify devise config/initializers/devise.rb and add this:
Devise.setup do |config|
config.authentication_keys = [:username]
config.case_insensitive_keys = [:username]
config.strip_whitespace_keys = [:username]
The final step is update the view to accomodate for username app/views/devise/sessions/new.html.erb