no no no no no
first of all the opposite sign of mercury is jupiter which rules Saggitarius. saggitarius being the opposite sign just have a different approach, while gemini's are curious fast thinkers and concern themselves with things like math/computers/problem solving etc.. Saggitarius on the other hand are more concerned on the big picture, about life, for example, religion/philosophy/theories/supernatural. that makes them philosophers and seekers of truth
pisces tend to be spiritual and have a hard time grasping things, not because theyre stupid but they just think a whole different way. they have good imagination.