The origin and history of this game world is different than the SRD while a lot of naming conventions will carry over some aspects are call backs to earlier versions or completely different such as Half-Elf and Tiefling having their own cultures and being products of unions between their own kind, yes it would be possible to have a Half-Elf child from the union of a Elf and a Human provided Humans existed in this setting. They do in the sense that at times a Tiefling , Half-Elf, or some other might have a genetic throw back that is mostly human in all discernible ways.
Ability Score Increase, I will ask that out of the listed for the Species that at least one is used without change as a way of representing that while not all are the same within a species there are commonalities. Siblings might be taller or thiner than cousins within the same family yet tend to have some qualities alike. As an example while I can not grow the same beard at 45 my brother did at 25 we both have a wit sharper than a shovel, that we use to dig our own … anyway.