yes you can.HEIDIY answer your question.

You can install plugins on a .siterubix site, you don't need to wait till you are a Premium member, you also have access to 12 themes in Wordpress. Once you are Premium you can access hundred (or is it thousands) of WP themes.

Go to your Wordpress Dashboard, on the left hand side you will see Appearance, under that is Themes. Plugins in the same location, you just need to add the plugins you want.

Hope that helps.

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You get access to heaps more Themes as a Premium member, not sure that WA recommends any in particular.

Most of the themes have an upgrade/Pro version you can pay for. I haven't upgraded my theme and it's working just fine.