As mentioned, the fasting makes sense. THe only surgery that any of my chins have had to have was for an eye laceration, and she ate that morning, don't know about that afternoon.
Nicodemus will do fine with only three legs. A friend of mine has a tripod and he is doing great. It will take a little time for him to adjust, so you should keep him in a smaller cage for several days after the amputation to give him time to heal. After that, introduce him to larger cage and heights in small increments so that he doesn't fall and hurt himself.
Now, for the Baytril issue. Not too long ago I posted/preached about the use of Baytril. Many vets overuse Baytril and it can cause harm/death if not administered properly. I haven't been following threads too closely, so I don't know how old Nicodemus is, but if he is under 18 months, I wouldn't recommend it normally. There are situations however which call for Baytril, and surgical procedures are usually one of them. Since my ragging on Baytril, I have had to treat with it on two occasions and both times there were no complications other than loss of appetite which required hand feeding. My recommendation is, if the vet feels that the situation warrants the use of Baytril, then do what the vet says to do. My only caution is, make sure that the vet doses the Baytril based on his weight. I am not a vet or a vet asst or vet tech, so what I'm telling you is based on experience, a great breeder and two great vets whom I trust my chin's lives to.
I wish you and Nicodemus the best and hope that his surgery goes well.