As long as you remember to designate the heir as the first child born into the house, I suppose what you're asking is okay.
Well, would any existing rule be violated by this? No Genie can be a household member is the main rule.
I would have thought that so long as the genie didn't move into the house then it would be okay for a spare to marry one. I don't see that it would be breaking any rules.
Rachel, if your Spares moved out to be with the Genie then whatever they did would be out of control as it would be dictated by SP?
A female Sim would be able to get pregnant under my control then move out before the baby arrived. I assume that would be okay.
I thought there could be no children at all with the original founders.
I always understood the rule to mean that the starting household had to have no children but you were allowed to marry a Sim from a household with children in. You yourself Pam had Christopher Steel marry Zelda Mae who comes from a household which has children. Obviously the children aren't hers but the rules says to pick a townie household without children so you would not have been allowed to use Zelda Mae's house as your starting house.
Edit: I think moving in children from a spouse's previous relationship would not be in the spirit of the challenge. But I do believe that marrying someone who already has children or comes from a household that has children should still be allowed so long as the kiddies stayed with their other parent etc.
I hope that made some kind of sense.
Rachel