Monsters Not Dropping Mesos

Only familiar rank changes, the level of the monster just effects the mobs it can hit and with what damage.

You can still use a snail at lvl 200 and still be able to hit anything at Stronghold. You just need to be the same lvl or higher than the familiar and you can use it. You would almost always find higher lvl mobs doing greater damage though (not sure if you want to count mutant ribbon pigs that can do 50-60k x 3 per mob which is a different approach).