645 is a good format BUT not the best if you specifically want to shoot with a WLF. The 6x6 was used with cameras with waist level finders in part to avoid having to turn the camera. With a square image there is no horizontal or vertical orientation. Depending on the film path and the format, a rectangular format will either shoot a vertical or horizontal from waist level, but you will have to turn it very awkwardly to shoot the other.
12 exposures is not a huge limitation, at least for me. In fact I prefer it to the 36 exposure loads of 35mm, but that's me and I'm not that much of a street shooter. The comment about inserts or backs is on target as loading does take longer (without preloaded backs or inserts) and you get fewer shots. For specifically shooting with a WLF though I'd recommend one of the 6x6 SLRs if quick film loading is needed. That would be the SQ-B among the OP's original list.
This doesn't apply to the RB with its rotating back, but as mentioned that's a big monster of a camera for street shooting. If you haven't seen one I'd not recommend buying one for this application until you see one in person. They are seriously big for any hand holding, much less street shooting - but some people do make it work. I wouldn't count on being unobtrusive with one though, regardless of finder!