Same plug (I own both T300 and T500)
In fact, the T300 (and TX) has a mode button to switch orientation of pedals, something which is only relevant for the T500 pedals.
Thrustmaster offers their own T3PA set, which has the same plug as the TX, T300 and T500 pedal sets. And there are even 3rd party adapters which convert Fanatec or Logitech G27 pedals into the standard Thrustmaster plug. And all options are confirmed to work on the Playstation(T300/500) and Xbox(TX) as well as PC.