I haven't had any problems. From my understanding, your apple id is the foundation. Your cloud email helps you identify with other apple users as well as use your cloud around the world. First make sure your iMessage is enabled in settings. Go to messages and enable. Then, there is a blank that says receive at: I put my cloud email here, and it works flawlessly. I believe if both parties have iMessage and it is enabled, your communication lines will automatically show in iMessage.
Disclaimer: phone numbers not belonging to iPhones will neither be able to send or receive messages via iMessages. If sent from an iPhone, they will revert to text. If sent from an iPod/iPad they will not send. They will error as unregistered number.
The iPhone doesn't have a separate widget for iMessages, your phone automatically sends the message to which system it belongs.
Just make sure you have the appropriate reception email in your settings.