Stuart's anthropomorphism is the result of lab experimentation.
- Well, scientists have using white mice in experiments for years.
- His parents were the ones who were experimented, and passed it onto him.
Snowball and Mr. Tinkles are from the same litter.They're both the same breed, and have the same voice and temper.
Humans can understand Snowbell when he talks.The Littles just ignore him most of the time because he tends to be a whiny Drama Queen. I thought this because of a scene from the end of movie 2 - when the Littles are headed home, Snowbell whines that he's sick of never getting any attention and claims that he's not returning home with them. Cue Mrs. Little calling out to him (in a perky but slightly annoyed tone) that she'll make him dinner when they get there, prompting him to join them. Additionally, Margolo can talk to humans and no one seems to bat an eye about it. It's possible that sentient talking animals are commonplace in this universe.
- He was somewhat right about being annoyed in that movie, though. For one, despite having chickened out at some point, he was instrumental in both Stuart and Margalo being alive by the end of the film by finding Margalo trapped in a paint bucket in the top floor of a skyscraper and releasing her, when he went with Stuart to look for Margalo. He also was right next to the Littles, just outside the taxi, when they were out finding Stuart, not even paying attention to their own cat. Not to mention he was there because the Falcon had thrown him down from the skyscraper inside a paint bucket.