Hey Li-An.
Does the plugin uses it’s own database table ?
There is one custom table created for the post cache, this table will however not be used at the frontend of your website. Related Posts are fetched with normal WP_Query objects.
Is there any way to custom CSS ?
Not yet via the plugin, this will be added in future versions. Elements do have classes so you can style them in your own/theme CSS.
No thumbnail?
Thumbnails are automatically shown if attached to the related post as featured image.
Do we have to redindex?
Related Posts for WordPress indexes all your posts on install, this will be done automatically in the wizard.
Please let me know if you have any more questions ??