I'm not a "knowledge base" on the expanded universe, lore etc. I've read my fair share, but there are plenty of other people out there who do know better.
However, as far as I can see there is nothing in lore that prevents any combination of species/class from the current choices?
Sure some of the combinations are far more likely than others. Some would only happen rarely. But as far as I can tell there is no combination which is actually a definite no. Am I right in that assumption/statement?
Short Answer: You are correct.
Aliens can do what they want. Just because a whole species feels a certain way doesn't mean that one character has to. Anything is possible in a galaxy the size of SWTOR, there are bound to be exceptions.
Long Answer: Ehhh.... kinda.
A few of the glaring Lore-bending combinations:
Are any of these or other combinations a "definate no"? No, they could happen. That character would probably just be its own special snowflake. :3