I can't think of much add that has not already been said on the subject...
Taking medium-format folders for what they are, I think they are probably best used for Group portaits or full-body portaits.
"tight head shots" are probably best done with an SLR.
For whatever reason(s), I've never been crazy about square format... although it's served Rolleiflex and Hasselblad well for well-over 50 years...
If I were to work with 6x6 ( I do have a couple of TLRs and a 6x9 folder w/ 6x6 mask), I would probably wind-up cropping to a rectangular print.
I prefer 6x9 on 120, but have just started taking pics with my Ikonta 521, which is a 4.5 x 6 cm format - rectangular image, but 16 shots on a roll of 120.
If you're really set on doing "real " portrait photography with a folder, you may want to consider a 2-1/4 x 3-1/4 Speed-Graphic, equipped with a 6x7 or 6x9 roll-film back...
This would give you the RF, interchangeable lenses, etc.
The blessing of MF is that you can enlarge & crop-down quite a bit before resolution / grain become a problem...
Good luck !
Luddite Frank