The Better WP Security plugin has a database cleanup utility that can delete autosaves, revision history, and drafts.
But--this isn't something that's necessary to do. A database is designed to hold lots of information and read and write to it in a quick manner.
And if I want to add addy custom databases to my wp theme. like for a game for example or to display a series of images...just add the new table in this database?
You really shouldn't be editing the wordpress database directly.
If you need to include additional data with your posts and pages, you can add custom meta fields.
You can also create your own Taxonomies and post types.
For displaying a series of images, use the media library's features. Or, if you really need some advanced functionality, the NEXTGen Gallery plugin is reasonably popular.
Wordpress is a very powerful platform...there is very little (if anything) that you actually have to do manually.