Wire size and breaker depends on the loads connected.What gauge wire did you use for GRID? The manual states 8 AWG but, a 50A breaker usually requires 6 AWG, does it not?
I too have a 50 A breaker in my main panel tied to GRID. Charging from GRID works fine other than a hum that occurs when the changeover occurs from GRID back to inverter on a couple of devices. These devices are two battery backups that contain low impedance transformers in them. Other's have noted a similar hum and there are a couple of threads on it.
If inductive/motor loads are the reason for the breaker size, the breaker can be 125% of the conductor rating due to nuisance trip prevention.