50 champion generator plugged into a Generlink 40 Amp Meter Mounted Transfer Switch
Doesn't appear to be a problem.  Supposedly, UL approved the instructions along with the device, and the instructions plainly invite you to plug in any generator with a compatible socket.  

The instructions state the meter device has internal overload sensing/protection, and if your generator delivers more than 40A (because you drew that much), it will automatically disconnect the generator for you.

This doesn't even require any extra hardware; presumably if you overload the generator, it simply throws the transfer switch to utility power (which is dead, obviously).  Rather clever.