Not really sure of your question but I'll take a stab at it. Running a skimmer is better than not having one at all even if the placement isn't ideal, but having it in the return chamber will cause you more headaches than is needed causing you to become frustrated with the skimmer and the hobby. So I would either have them build your sump so that you have a different place to place your skimmer or purchase a skimmer that could be placed in one of the other chambers. If you can provide a description on how your sump is being built we can give you a better idea on what your options are.