The Squirtle event is one of the few outdated things on the wiki.
It needs to be raining for the guy in the building to appear, but then it needs to be clear weather for the Squirtle to appear. Also, you can get the Squirtle without the Tauros, in fact you can get it right after defeating Aya.
Use this very helpful guide to get the directions to it (Ctrl+F for "FSquirt1"):