I am not sure understand what you want.
1. Create a custom.css file in template 'css' folder, and it will be load automatically (please do not modify the head.php, there a rule to load custom.css file in this file).
2. A module can/should have their stylesheet. Just use official "JDocument"->addStylesheet method to add your stylesheet for module.
3. You can always add any other style sheet by modify head.php, you can use addStylesheet via PHP code or <link .../> tag