If you really want to cram things in to a 5' square, look in Complete Warrior at the Swarmfighting feat. It lets multiple creatures of small size stand and fight in the same 5' square, with no stated limits as to how many. Unfortunately, the same feat appears in Races of Faerun, and in RoF they specify that you can only fit 4 small creatures in a 5' square (and creatures one size category smaller than small count as half a creature for this).

However, that brings up another dysfunction. The RoF Swarmfighting feat says that "Up to four creatures of Small size may occupy a 5-foot square (creatures one size category smaller count as half a creature)." Two issues: It doesn't state anything about creatures two or more size categories smaller (do they all count as full creatures? By RAW, yes), and second the prerequisites state that you must be "Size Small". Not "Size Small or smaller", just "Size Small". According to that, creatures smaller than small cannot get the feat, yet it specifically states how the rules work for creatures one size smaller than Small.