I am planning a wedding of approximately 150 guests. I know that you are supposed to have one usher to every fifty guests. Do those ushers also have to be groomsmen?

 No, they do not have to be the same people. You can have separate ushers whose only job it is to escort people to (and maybe from) their seats. This is one way to have friends/relatives participate in your wedding without having them in the actual wedding party. But, if you choose, they can be the same people. After the guests are seated, they then take their places as groomsmen.