Question: If you are building each of your founders a house can you spend a few rotations doing that. In other words... is it ok that the houses take a few sim weeks to build or must the houses all be completed on that first day?
Yes it is okay to take a few sim weeks to build your house. Your sim will just live partially out in the wild while you are remodeling/building your house.
Also must I get all three founders a career on that first day? I was planning on getting them jobs at the end of their first rotation so I have time to do other stuff with each of them. Sims don't actually earn money when you are rotated away from them, right? So I don't think not placing them in a career benefits one bloodline over an other. (I think.)
No, you do not need to get a career on the first day. Maxing a unique career is a requirement though, you might want to start early if you have trouble maxing careers.
Also I'm a little confused why there is such a rush to move the next heir out etc. After all your sims that you don't play anymore won't age/die... I think my strategy seems really different from anyone else and I'm starting to think I've missed something!
The rivalry is basically a race. Which bloodline has the most lots win, so you want to rush them out (at least that's how I play it). You don't have to rush them out if you don't want to, take your time and enjoy writing the story, but still at the end of the dynasty whichever bloodline has the most lots will be declared the winner.