Monsters Inc takes place before Toy Story. Randall from Monsters Inc has Andy's clouds wall paper as something he needs to match in order to blend in. In Toy Story 2, the clouds became stars being that Buzz Lightyear is from space and is Andy's new favorite toy.
Sid's room can be seen in Monsters Inc being Sid's Monster is Claws. Claws comes out from Sid's room shaken and let's out that a little girl almost touched him. Claws was trying to scare Sid but didn't know that it was Hannah. The family was in the process of switching rooms so that lead to the mix up. The poster in Toy Story is blue because of age do to the lava lamp that is right underneath it while the poster from Monsters Inc is green. Hannah in Toy Story looks like she is about ten and Monsters Inc places the girl at six. Andy turns six in the first toy story so Andy would be about Boo's age which is two.
We can rule out that the little girl that's in the daycare holding the purple cat as Boo. My thought is that Sulley is her monster now.
Boo has some of the same toys as Andy such as Jessie and the Pixar ball. I know Pixar put the ball in every movie.
In Toy Story 2, Woody didn't know about the show Woody's round up so that would suggest that Woody was a reproduction not an original which would explain Jessie on Boo's floor. Buzz tried to contact star command after coming out of his box which suggest that toy's knows about their backstory, if they have one. A reproduction toy wouldn't know their back story.
This one is a bit of a stretch. Bonnie (Toy Story 3) is Boo's cousin and Boo lives with them being of the Sulley style toilet seat cover. We don't see Boo's parents in Monsters Inc so that we can assume that they are dead. Boo would have fell in love with the cover being it looked like Sulley.
What about Andy's door in Monsters U? The time in the monsters is different from time in the human world.