Pekins grow super fast, so make sure you get started right away on predator-safe housing for them. I would also make sure they're being fed a duckling specific food, and add nutritional yeast or brewer's yeast as well - 1 Tbsp. per cup of feed. Pekins are bred to reach harvest weight at 8 weeks and the stress on their legs more often than not is too much - their bodies need a lot of extra niacin. You should just go ahead and get the liquid vitamin B complex as well and give this to them over their feed or treats, 1 ml per duck per day. That will ensure your babies don't run into debilitating problems, which are so common this time of year when people get pekin ducklings.
I keep my ducklings inside until fully feathered. Make sure that once they're big enough not to slip into and drown in water, that they have water deep enough to dip their whole bills into and clean out their nostrils. Also, as you start to understand how many males versus females you have, know that usually the ratio to avoid over-mating is one male to 4 or 5 females. Also, male ducks mating with chickens can injure them - so you want to make sure you don't have that situation. So have a plan for your males - when getting mixed ducklings like that you will likely have more males than for the proper ratio - you may need to build an extra area onto your duck coop to separate males.
And then what I would say is different about ducks is that, especially pekins, are very clumsy compared to chickens. They can't walk up ramps, only the shallowest of inclines, and where chickens like dry things and scratching in the dirt, ducks like it wet and really need water to swim in to be the most happy, and they will make things muddy.
With Pekins you especially need to be aware of bumblefoot due to their weight - check their feet once a week once they are grown, and make sure their surroundings don't have anything sharp or pokey that will create cuts on their feet.