Honestly, the idea of wiping out the lizardfolk just to use them as undead seems contradictory to Lord Ainz's original plan. Sure, they will make do as tougher zombies, but I thought Ainz was trying to keep a lower profile of Nazarick's activities; wiping out a settlement of people, human or not, will not accomplish that. And 8 days' warning? That is really stupid; what if the lizard people just decide to scatter into the waterways and flee within that long period of time? It's better just to go with a surprise attack to prevent the escape of as many as possible.
Of course, given the writing is as it is, no doubt they'll just hang around and wait to be slaughtered.