So it basicly comes down to lore context clues and companion dialouge i have beat every story atleast once so i think i have got them down pretty close.
For knight - id say human fits the best it doesnt need much explaining basic jedi nothing special cliche hero type.
For counselor - i did mirluka first then cathar, being honerable and all that but mirluka is a deffinate best fit for sage and possibly togruta for shadow once it comes out.
For trooper - rep zabarak most people went into this already so you can just read that.
For smuggler - any default race smugglers are just criminals and come in all shapes sizes & colors.
For warrior - pureblood works best for voice & purity sake it talks about how your groomed to be sith and a perfect wepon & what not.
For inquisitor - it has to be human first of all there are tons of human slaves & you join the dark counsil before darth karrid & she lorewise is the first ever alien on there and kallig your ancestor was alive before most alien races in game were discovered and its obvious through his mask hes not twi'lek
For BH - it doesnt matter as long as it isnt a force sentric race
And last but not least agent - chiss most dialouge options best story fit , empires allys , voice fit & the race was made for this class specificly not to mention every bit of promotional media shows him as a chiss , doesnt matter much but still worth mentioning
Obviously biowear wants people to deffine their own char but they clearly had a certain race in minda for each charecter but well honestly probably never know for sure