For printing ratio, 6x7 negs can print full frame on 8x10, but crop on 5x7. I love square format 6x6, and trim my paper to suit my needs. I also shoot 4x5, so enlarging to 8x10 full frame, no trimming. My 35mm (when I print any) enlarges to around 6.5x10 full frame, and I trim the paper. Decide which format you really want, not byconvenience of paper sizes, but by what fits your desires and vision.
Pentax 67's are monsters, and aren't really hand-holdable. 6x6 TLR's are hand-holdable, and quiet(no mirror slap) I have hand-held my Kowas, and they are not too awfully heavy or awkward. Methinks the Bronicas would be similar(SQ for 6x6, ETR for 6x4.5)except the GS(?) 6x9 system.