It would seem logical that a mon cal cannot wear the same helmet that a Rodian or a twi'lek can wear. My guess is that you will find that most armorers can fit just about any race. It'll be something like, they can make helmets for this race, this race and this race. That sort of thing, but it's only my guess. Seems logical though.
I think ive seen sum where that the helmet/whatever is adjusted visually so that it fits ur character. Like a mon cal helmet, i think ive seen the graphics 4 that, and a chinstrap or sumthing is removed to make the helmet fit. Don't have a link sorry.
I think any piece or armour will be able to be worn by every race. There might be special kinds that are set aside for specific speices but in all the interviews and FAQ's they said they want everything to be accessible no matter what skill, race, or amount spent playing the actual game. Well thats what im guessing anyway.