Have you studied the book "A first course in GR" by Schutz? In chapter 4, he discusses the perfect fluids in SR which is catchy if you are interested in learning beyond pressure, something like the components of the energy-stress-momentum tensor (e.g. pressure, density and non-gravitational force fields) can be well pinned down via his book at first glance if you have a good knowledge of calculus/SR.
Well, I don't think the strong force has been combined with the other two. There is just an electroweak theory which covers electromagnetism and weak force together!