Just to toss in something else. We have a Mon Calamari companion for Smugglers. Mine wears all of my hand-me-down armor and it works just fine. They simply do not show the helmet or gloves. Everything else works.
I would think that the lack of being able to squeeze their head into a standard helmet shouldn't bar a species from consideration. Ditto the hands and even the feet. I'm sure most people who want species that differ in head/hands/feet would be content to go without helmets/gloves/boots to have them.
I could be wrong, of course. :-)
I am with you on that