In the menu, System - Preferences -Keyboard will enable you to add Tamil. Click on Options on that tab and you can set the key combination to change layouts and set the Scroll-lock light to show when the second layout is in use.
Right click on the panel and you can add a keyboard indicator to show your layout with a name or a flag. You can also use it to change layouts by clicking on it.
Red Hat will have Tamil fonts, but there are more here:
If you get a third-party font, put it in .fonts in your home directory and it will be immediately available.