If at all possible, please please please fix it so that if Overstuffed Houses is enabled, sims can still have twins/triplets after there are 8 people in the household.  Right now, you can only have single births no matter what after you hit the 8.

Better yet, a tool like the Sims 2 Twins/Triplets/Quads mod where you can choose how many babies you want with each birth.  Thanks crazy much in advance for either!!

LOL!

The problem is that they took away the option to just age up.  So if you don't want to wait around until 8/9 pm of the last day and have it interrupt whatever they're doing, then you have to use a cake.  I see the point though, how then would you distinguish when you DO want a party with everyone in attendance.  Although, people who have chosen to go to bed wouldn't typically wake up to go watch the candles being blown out...

Having this feature enabled stops Sims from dropping everything when someone grows up except if they are sleeping.  Sleeping sims still wake up to go celebrate the birthday.

While this mod allows more than 8 in a household, if you're getting more by giving birth, you can only add one at a time.  I have the overstuffed household enabled, and I've tweaked the XML for determining chances of getting twins/triplets.  One couple had triplets with the first two births (female had fertility treatment on first birth, both parents had it on second birth, and the mother didn't even watch more than about an hour or two of the kids channel during the first birth, and as mentioned I have tweaked the chances).  At this point, there were 8 in the house.  She WAS able to get pregnant again, but after multiple hours of kid's tv and radio, there was only a single birth.  After more extensive testing, I've found that it just won't let you have multiples after you've hit 8.  I don't know what kind of check it's looking for or where the parameters causing this functionality are located, since it's functionality that wasn't originally intended by the core game.

Do we have any idea what controls the Sim portraits and keeps them showing only 8 even with the restriction on actual household members lifted?

I'll definitely grant the Prima guides aren't the final word on these things, but how about EA's own coding?  Here's what I found while poking around in the gameplay packages:

<kGenderOffsetPerFoodEaten value="0.15">
      <!--Offset provided to gender random per food eaten (watermelon & apple).-->
    </kGenderOffsetPerFoodEaten>
    <kMaxGenderOffset value="0.5">
      <!--(0-.5) Maximum offset from .5 allowed to be achieved.-->
    </kMaxGenderOffset>

And regarding multiples, that experiment about watching kid channels may have some merit.  Although it still looks like your chance of triplets is still only 3% if you watch kid tv and listen to kid music for 6 sim-hours (whereas your chance for twins is raised to 75% in the same situation, which is pretty cool).  I'm assuming this is for regular sims without the fertility booster; will update if I find that coding.

<kChanceOfTwins value="0.025">
      <!--(0-1) chance of getting twins.-->
    </kChanceOfTwins>
<kChanceOfTriplets value="0.001">
      <!--(0-1) chance of getting triplets, must be less than chance of twins.-->
    </kChanceOfTriplets>
<kMaxBabyMultiplier value="30">
      <!--Maximum multiplier used when determing the chance of multiple babies.-->
    </kMaxBabyMultiplier>
<kKidMultiplierPerHour value="2">
      <!--Multiplier added to base per hour of kid entertainment (tv or stereo).-->
    </kKidMultiplierPerHour>
 <kDoubleKidMultiplierPerHour value="5">
      <!--Multiplier added to base per hour of kid entertainment (tv and stereo together).-->
    </kDoubleKidMultiplierPerHour>