Zombieland was originally imagined as a TV series. However, when all the networks the creators went to shot it down, eventually the creators settled on making a movie out of it. The "Zombie Kill of the Week" is pretty clearly a holdover from the TV idea. Ironically, when the movie was proven a hit, Rhett Reese and Paul Wernick were considering in making a TV Spin-Off that never came to be after a failed pilot on Amazon Video, in 2013.
In the DVD extras, it's revealed that Abigail Breslin liked the zombie makeup so much that she asked the producers if Little Rock could become a zombie at the end. They turned her down.
The sequence with Tallahassee making a last stand is riddled with continuity errors. Look closely and you'll see that he starts out with a 1911 and a P226, then midway through the fight the Sig changes into a P228, and by the end of it he's dual-wielding two 1911s. The whole scene looked cool, so nobody cares, especially when you consider that having more than two guns on handwould be perfectly in character for Tallahassee.