But as far as I can tell there is no combination which is actually a definite no. Am I right in that assumption/statement?
You are correct, there is no combination that is, by lore, impossible.
It boils down to likelihoods. Like Warren-Stride showed, there are certain race/class combinations that just do not make a lot of sense.
I play Dungeons and Dragons tabletop RPG, and in those rules it says that any race can play any class. However, in each class description there is a section about what races are more likely to be said class. The same thing can be said of the star wars universe: all races can be all classes, but each race has its leanings.