Note that most of the time any combination of a mermaid, vampire, alien, magic user or sim can have a kid and it will just turn out to be one of the 2, for example not an alien vampire from Outer Spaaaace!, just an alien or vampire. Although kids of aliens often turn out blue or green, so you can have a vampire who looks like an alien vampire from Outer Spaaaace!
The problems come in when the game glitches out or you use cheats or mods to create a sim with the powers of both its parents.
EDIT: And the best way to learn building is lots of practice. Everyone starts off making boxes, unless I'm trying to recreate a plan my houses still look like boxes.
Pick an empty lot from the town overview (not the lot your family is living on, that adds pressure and costs money) and just play around in free build mode, getting the feel of the building tools, don't actually try building any thing, just get the hang of the tools, how the room and wall tools work, how roofs work and how you can combine them to cover an odd shaped house, that sort of thing.
Then try building something simple, doesn't even have to be livable or good looking, just something to test what you know.
Also, a random hint, the Christmas trees can catch fire when you light them.
