Starting them at the same time isn't the best idea. Welsh Harlequins take the same amount of time as any other mallard derived duck - 28 days. They lock down on day 25. The problem with staggered hatches in the same incubator is at some point you'll have to jack up the humidity and stop turning the eggs, which isn't good for the eggs that still need lower humidity and to be turned.
I do incubate duck and chicken eggs at the same time all the time, but that's because I have a separate incubator to move the eggs into as they go into lockdown. The best way to do this in one incubator is to set the ducks eggs, wait a week, and then set the chicken eggs.
I'm not saying you can't do it the way you're planning, but it will make a lot of extra work for you and your hatch rate may suffer.