As noted, Tieflings have been part of D & D lore for decades now - in fact one of the potential companions in BG1/2 is a Tiefling Bard. Their presence in larger numbers is again due to current D & D lore, involving Elturil. However, Larial could certainly throw in some in-game conversation content in which the Player can obtain more detailed information about this race from Volo, druids, tieflings and even shop keepers. They could even sprinkle a few cooks containing relevant lore around the grove 0 again maybe Volo has been putting something together you can look at.
Then let's now compare Haer'Dalis with Zevlor:
-> of course there will be no problems coming across Haer'Dalis and having him act as normal human afraid of Goblins. In the case of Zevlor & Co, it's weird that the Goblins are not afraid of him.