At its leanest and meanest, the Reformed package requires some variant of Calvinism, plus Covenant Theology. Supersessionism and/or postmillennialism are often associated with these but are not, so far as I know, mandatory in order to call yourself Reformed. Some of these concepts fit together better than others; for example, supersessionism harmonizes quite naturally with CT. If there is only one covenant people through all the ages, it follows that someone has to be in and someone else has to be out.
Not all these theological components fit together quite so well as that pair.