I thought that all MTB & road bike derailleurs would work with whatever shifters, until you got to 10-speed (or 9?).

I've had road shifters with tourney and lower, and claris derailleurs. Maybe the other way around, the trigger shifters are different, but am I wrong that the pull ratio is the same on 7 and 8 speed road and mtb components? In that case it would work if the DR total capacity is enough.