If you are comfortable editing components you can adjust existing components to whatever you like. Edit the 10x10 and push / pull it to an 8x8. You'll need to adjust the tenon, housing, etc. as well.

If you are new to the TF Rubies and modeling in SketchUp I would recommend spending a few hours and creating your own library of timbers. It is quite fast once you get started. Start with a 4x6 and work your way up. Make the 4x6 8' long, copy it over, make it unique. Edit and change it to a 6x6. Repeat on up through your size combinations. Be sure to change their names so you know what they are... you can use Window>Entity Info to do this. Then right click each component and 'Save As...' to your local joinery directory.