the specification you are quoting is input sensitivity which is the nominal voltage input required to achieve full rated output from the amplifier.
the spec you are looking for is overload capacity, that is how great of a signal the phono input can tolerate before overloading. this is specified in the MA-5100 manual to be 125mV, which is higher than any cartridge you can think of will output.
I have used a Shure M7D with 5.0mV output into the MA-5100 phono section with great results.