S.C. Test on a transformer
while you model the impedance of the transformer as a constant(R+jXc) it really isn't and depends on the magnetic field that the transformer produces. Ask yourself what is the Xc when the transformer is saturated... The inductance in the transformer is not constant.

So you want to  build a model that works for rated current, sure if you stay within the limits of the field for hysteresis the losses and field will not change much so even the "rated current" model might give you a good description in this condition.

Yes, they can be measured easier by a multi-meter but you will not get a full model this way, the parallel resistor to the winding is usually to model the eddy current losses, not the winding resistance itself(You are not taking that into account right now I guess)

if you purely care about the actual resistance of the winding, a multi-meter will get you that reading, I am just trying to remind you that is not the only loss in the circuit.However, when using a multi-meter remember that it feeds DC to the transformer which can saturate it very quickly leading to poor measurements.