I support the use once, and I support the killing entire population thing, although I think that it should be a value you can add in that kills a certain percentage of the population. Most people would obviously place this at 100%, but for some units other than nukes (Long range missile for example) you may only want to kill 2% of the defenders. the percentages would stack with each unit that is using this that is added into the stack. For example, 5 long range missiles at 2% each would kill 10% of the defenders if all used on one city.
As for the fallout, it would be more realistic but I don't know, seems kinda OP.
----
"Bitches ain't shit, but hoes and tricks"
-Mahatma Gandhi