So if I'm following the thread we should use something with a more substantial sole then the Tigullio's that hubby & I Both have for our open heel fins.... yes? or are they okay?
Maybe. It depends on how soft the soles are and how tough your feet are. My wife wore something similar for years and walked around with tender feet on most trips.
... alternatively, if we only bring our existing booties, are hard soled ones available at dive shops on BON?
Yes, assuming they have your sizes in stock. My wife bought her Seasoft Sunray booties at Bruce Bowker's Carib Inn dive shop on Bonaire. She paid the same price as I paid for mine at my local dive shop in the USA ($70 in 2010.)