I have set up a rule when I DM that everything in regards to race and class are possible, but anything outside the core books (and certain books other books that I specify at the beginning of the campaign) has to be ran by me so that together me and the player can decide if the race / class is broken and if we can fit it into the campaign world. Most of the time it is possible to do, but sometimes we have to tweak a few things to make it either not be overpowered or to fit in with the setting that is being run.
