I've taken courses on supersymmetry and string theory and this is mostly incorrect. The details are fairly complicated, but the general idea is that there are a

I've taken courses on supersymmetry and string theory and this is mostly incorrect. The details are fairly complicated, but the general idea is that there are a few essentially unique supersymmetric theories in 10 dimensions that can be "compactified" to a vast variety of effective theories in four dimensions. From a purely phenomenological point of view that can be ignored however and people have come up with various supersymmetric extensions of the standard model of particle physics in four dimensions going by names like MSSM (minimal supersymmetric standard model), basically introducing aditional superpartners of known particles by hand (like the selectron and photino you mentioned) and "completing" the model by computing the deficit for it to be supersymmetric. Those completions are not unique and can be distinguished by the energy scales they introduce new particles at. One major motivation is to constrain the running of the Higgs mass, which sets a natural upper bound for the expected masses of supersymmetric particles to be found. Those bounds have already almost been excluded by the current data, which is scary or exiting depending on who you ask.