gar might be okay they are generally top dewllers. as for scales being hard gars do not have hard scales. and an aba's bite can go though bone. I would say try it and hope to have a shy aba, and a passive gar. and keep a spare tank in mind too.
aba aba's are great looking fish I loved mine. im not trying to discourage you just trying to educate the subject. mine ate a lot of expensive fish and I hope you will learn from my experience.
i had one for a short time it was in with gars and birchirs very agressive towards tankmates funny enought my lap ate it kill or be killed very agressive keep it alone no tankmates