@yourrid in lr-genesis-plusgx you are supposed to be able to switch between 3 and 6 button with the select button but it doesnt work. And strangely some 6 button games default 6 buttons and some dont. So i just use lr-picodrive. Problem solved.
Yes - the 6 button controller for Genesis can be used with the GamepadBlock. You can switch to 6 button mode with the button combination "A+B+C+Start+Up".