Well they altered them all to be different at one point. think at least one can cast magic but im not sure its been a long time. Metal slime has like low HP but mega defense or something. There is also the spriggans by the way. I didnt really have any problems leveling normal slime (least to 30 or 60 however far i got it) when it first came out even without really awesome instincts to set. it just was kinda boring cause it could only melee back then.