the giants did it the right way, pitching their guy with normal rest for two starts, then bumping him in for however many you could get from him in game 7 (if needed). handicapping your best guy in one of the two games he's going to pitch anyways doesn't make any sense.
if the royals don't feel like they have a 4th starter worth crap, then it'd make more sense to make game 4 a bullpen game and then roll back into game 5 assuming cueto can/will have to go 7, 8 or 9 and get them back rested
alternatively, you could get creative and go with three starters each going 1x through the lineup with duffy, young and medlen/ventura. then your one inning bullpen guys stay as normal. lots of options better than asking cueto to do something he's never done before (he's also never been good in the playoffs before, so you're really asking him to do two things he's never done before).