Most wood stains contain a sealer in them. If you can find out which brand the contractor is using, you'll be able to go to a store to find out if it contains a sealer. IF the stain did NOT contain a sealer, the wood would have to be sealed AFTER the stain is applied. If it's done in the reverse order, the stain wouldn't be absorbed by the wood. You see, the sealer would seal up the wood pores.
Polyurethane is not a sealer as you're probably well aware. It's simply a protection over top of the wood to prevent/minimize physical damage to the wood from everyday use. My $.02 says that you ought to see to it that the contractor uses a stain that contains a sealer already in it. One way to insure this is to pick out the stain yourself. Bring in a sample piece of wood, or 2 or 3, to a paint store or home center to test the stain you choose. Wait a day for it to dry and sink in to be sure that you like the color.