skimmer pump too small?

I am going to be buying a new skimmer this weekend from a friend.question I have is the skimmer is raster for a250 gallon tank.I have a 90. He has it running on a 40 gallon. He put a smaller pump on it so it wouldn't be so powerful.is this something I should be concerned about? Should I put a bigger pump on it or is this actually something you can do?