Combining inverter and EV charger

Asking the inspector is always a prudent thing to do, my understanding is you want to tap into the branch circuit which the GTI inverters are connected to, which is protected by a 40A breaker. The purpose of the breaker is to protect the conductors, therefore all conductors in the branch circuit must be sized appropriately, a smaller gauge cannot be included in the same branch circuit without reducing the capacity of the branch circuit  breaker to the capacity of the smallest gauge, regardless of its location in the circuit. Including a local convenience disconnect with lockout provision for service is good practice, or may be a requirement, a circuit breaker or fused disconnect should  be utilized, which if limited to 30A, would allow a lighter gauge downstream however the distance would be so short the added conductor cost would be negligible. Some devices state a maximum breaker capacity allowed to protect internal circuitry, in which case the appropriate value should be utilized, otherwise I can't see why this application would be treated differently from any other branch circuit, with the exception of a short circuit at the charger, which would theoretically allow both grid and solars combined output to be added, as @BB. pointed out, this is where the fused disconnect/breaker would protect such an event. It's an interesting subject however without reference to the applicable codes these are just thoughts, which may help to rationalize your intent when asking your authority having juristiction, or your local inspector, if you like.