There are hacks available at MTS2 and
http://www.moreawesomethanyou.com which will remove the maximum sim limitation from your game permanently. Unfortunately, the latest word is that these hacks are not compatible with Pets, but the authors are working on them. If you own Pets, Micco's solution is the best way to go. The downside to the Summoner/Family Tree magic is that as soon as you eclipse 8 household sims, you will lose the option to Try for Baby unless you have one of the aforementioned hacks installed.