On duck is not a good idea unless you are planing on it being a house duck. Pekins are just like other ducks and NEED a buddy. If you are going to be that companion that is fine but you will need to be with your duck almost 24/7, ask @Amykins She has a house duck who is a companion (which we all love). But if you plan on your duck being with your chickens that is fine, but you don't want him or her to be alone. Ducks are not chickens and do have different needs such as water. You can raise them kinda together but ducks grow so much faster and can step on chicks or the chicks can peck and hurt the ducks with their beaks. If you haven't had either before I might suggest you get either chickens or ducks first then get the other later.
Ducks can have non medicated chick starter but do need more niacin than chicks so you will have to supplement or get an all flock feed. Brewers yeast is an easy way to add the needed niacin.
The actual coop is 10 ft by 5 ft? Ducks don't roost so they need more floor space than chickens around 6 sq ft per bird give or take depending on how much time they spend in the coop. Mine have less be there are never any days that they cannot or will not go out to free range my backyard. The more birds you have in a smaller area can cause healthy and behavior problems. Another book is Storey's Guide to Raising Ducks. It's more of a farm approach but is very helpful.
