So I'm starting to DM my second campaign this Sunday, which will be Rise of the Runelords, since I have people who want to play more however I'm unable to make content for my homebrew at a fast enough rate to play twice a week, so I figured we would play an AP as to cut down on prep time. That said my initial game I limited everyone to core only so that I could more easily help them and could easily learn their races and classes (they were all mostly new) however this time I decided to open things up more allowing any class and any race under 15 RP. That said it's clear some races aren't really liked, some like strix on the side of attack on sight, while some like tiefling and aasimar which seem like they are just treated different as is apparent in Nualia's story and the way she is treated .

That said dealing with the way these races are generally treated doesn't bother me, and for the most part my players have avoided these races as it either doesn't fit their style or the character they would make would be pretentious towards them also. I also am currently a player in a Serpent's Skull campaign where the GM (now regreting it) allowed a player to play tiefling after two of us had already selected a backstory basically as one possessed by a demon and her body guard who is essentially not into anything supernatural, so needless to say we are already on book two and apparently the tiefling still looks like a human girl to us and no npc has made any mention of her. We had a drow in the same respect as the tiefling who tried to hide his race and needless to say it was nothing but annoying when we found out, and for the most part our entire suspension of disbelief is currently lost.

Some of my player's in my new campaign are coming from the same game previous and I know a repeat is going to cause them to go through the same thing. The unfortunate thing is I have a player who wanted tiefling, but wasn't sure if he was okay with being treated differently, then asked the party what they thought and was asked if picking another race was possible. So he's now asking about goblin, and knowing how the story plays out that's probably not the best idea. I have since told him it'll depend on his backstory basically saying for him to convince me how he's realistically there and not going to be attacked. That said I really would not want to ruin the other player's suspension of disbelief just so we can have some crazy story about how this goblin is there and how even if this player can't roleplay it well you have to accept him into your party (I have polar opposite rolpayers, some who absolutely love it, some who hate it, and some indifferent).

I suppose my question is, am I being too harsh on the races with how they are treated, and/or should I just make it a flat rule to disallow races the party has already made mention they'd rather not have in their group? Any advice is appreciated, I don't like just ever say "I'm the DM so we are doing it this way" however the added complexity has only been really making things more confusing for some players and stressful for others.

EDIT: So I've since talked with the player a bit more, he's since decided to change his race to human. It seems there was a bit of a communication barrier (which I guess comes when you only talk though skype text) and he was fine with changing the whole time. I suppose it goes to show some of the simpler things of "talk things out in depth with your player" is perhaps the most important thing to remember.