1. I hadn't heard about the 100 episode rule, but most live action/primetime sitcoms apparently have to have a minimum of five seasons to enter syndication, and syndication ensures those involved in the series will keep getting residual checks, which is why once most shows reach Season Five, they go ahead and end it afterwards (though for some reason, the crapsack that is Big Bang Theory was the exception to the rule, and got a syndication deal well before they even reached a fifth season, now it's on, like, three or four different channels).
2. The 65 episode rule for weekly cartoon series apparently has something to do with spreading the reruns out evenly in a Monday through Friday cycle (like newspaper comics); depending on the distributor of the series, they may or may not purchase every episode in the series to syndicate it, but they have to purchase a minimum of 65. For example, shows like Alvin and The Chipmunks only had their first 65 episodes purchased by the distributors for syndication, even though they had more, the distributors simply didn't want to pay extra for them since they already met the 65-episode minimum. In other cases, like Rocky and Bullwinkle, there are two different syndication packages: one in under "The Bullwinkle Show" titles, and contains Seasons One, Two, and Five, while the other package under the "Rocky and His Friends" titles contain Seasons Three and Four, for whatever reasons.
3. Most cable networks that rerun older shows tend to exclude holiday episodes, except for special airings around the respective holiday season; TV Land used to do this with all of it's shows, you never saw the Christmas episodes, or other holidays episodes, except for special marathons in December. I think the only exception is if the episode itself isn't strictly about the holidays, for example, there's episodes of Seinfeld and Everybody Loves Raymond where the holidays are simply used as backdrops for the story, rather than actually being part of the plot, which is why you still see those episodes any time throughout the year.