How to use separate theme for a front page and quite different theme for the other pages?
You can try the Themekey module, that allows setting different themes to given paths (you can use the path to target the frontpage).
Instead of creating a completely separate theme, you can also override the front page template (page-front.tpl.php) in your current theme. I'm not saying that you should never use a separate front page theme, but overriding the template is often an easier solution with less overhead.
You can also use the Context module.