I am afraid I cannot answer all these questions. It seems that most of them are not about the use of Brian but rather on how to model your experiment in general (which, as I said earlier, would rather be a discussion to have with a supervisor or collaborator). A few comments, though:
* Are you sure that your STDP window is 12 microseconds? I don't think you can have a pre spike trigger a post spike with such a short delay. In particular, the Brian simulation uses a time step of 100µs, so all spike times are forced to that grid and you will never encounter spike time differences above 0 but below 100µs. You could change Brian's timestep by setting defaultclock.dt, but it doesn't seem reasonable to me to simulate at that time scale for a neuron with a membrane time constant of 20ms.
* I am not sure what you mean by the "initial conductances" for the left/right parts of the STDP curve.
* In your model formulation, ge and gmax are multiples of the membrane conductance. You could use a different model formulation where you have the membrane capacitance and membrane conductance as separate variables instead of a single membrane time constant tau. Then, you can set your synaptic conductances to use actual units as well.
Two general suggestions:
* Make sure that the model parts are reasonable individually before putting everything in (e.g. does the cell something reasonable without STDP, e.g. does it fire at all but not at every time step?)
* Do not expect to be able to directly put experimentally measured values into a highly simplified model without adjusting them. I guess your experimental cells do not have exactly 1000 synapses spiking at 14Hz either.