Questions on mini kossel belt system

Hi everyone, I am building a mini kossel. I'm generally following the instructions here:

[www.think3dprint3d.com]

I see that they are using two flanged bearings pressed together for the belt idler. Is this really a good idea? It seems to me that I should use another GT2 20 tooth pulley on the idler. My solidoodle uses pulleys on the idlers and it works fine. It seems like having a smooth idler would lead to slipping.

The GT2 20 tooth pulley is the best choice, right?

Also, is there anyone out there who has built a mini kossel with spectra line? I was looking on thingiverse and I didn't see any carriages that had been modded to use spectra line. I like the idea of zero backlash and I think I'm more likely to get that with a spectra line system, so if there is a design out there already, I would like to use it unless someone can convince me why I am better off with belts.

Thanks.