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?

Ta