Naming Categories (slugs) and Image Sizes

Hello,

Would you please advise if you can use the same category slugs for slidercats, portfolio categories and page name slugs? I don't know if these work independently of one another or not. I know in Wordpress you should not use the same category name as a page name as well.

Also what is the best image size for uploading portfolio photos? Please include both horizontal and vertical image sizes.

Thank you,

Deb