In episode 1.15, "A Pox in Our House", Joey claims that he has never had the chicken pox and is therefore immune to them. He then gets the chicken pox later in that episode. However, in this episode, Colonel Gladstone talks about watching cartoons with Joey when he was three and had the chicken pox. Joey's reaction ("You watched cartoons?") makes it clear he does not remember this, which is quite believable given that he was only three, and thus grew up believing he was immune to chicken pox. Also, it is possible although unlikely to get chicken pox twice.
Jesse says that he has known Joey for four years. This would put their first meeting roughly a year before Pam's death and the start of the show. However, it's been established in season 2's "Pal Joey," that Danny, Joey, and Jesse went to elementary (and possibly further on) school together. It may be possible that John Stamos meant to say fourteen years instead of four years. Which, even then, would be an error in continuity based on the above.
It's never explained where Michelle was since she wasn't present with the family in Las Vegas. She could be staying with her grandparents or Rebecca.