From what I can tell the life span issue only relates to Enzyme I and III, in I's case it was because he like the Test Type at the started of the series it hadn't been full optimised and so only had one week to live. The III's on the other hand have much shorter life spanes because of their healing factor, their matabalisame has been sped up so much to achive this ability theirr bodys burn out a humans 67+ year life span in just 3. Its worse for a zoalord becuase their bodies have been so heavly modified that with out the energy of a zoa-crystal to meet their bodies gigantic energy consuption they can die in a matter of days or even hours without medical intervention. Only two Enzyme II's had short life spans and that was because their optimsations hadn't finished when Guyot pulled them out of the tank.
As for running on impulse and instinct only the III's do this, Enzyme I was acting on pure rage but thats was because Genzo was so full of rage at what had happend to him he went berserk and he was able of thinking stratigicly as he was able to disable Sho's megasmasher at the start of the fight and in the manga and 2005 anime when Sho had healed and tryied to use it Enzyme stabbed him through the soft lense with his tail. While Sho's father acted very animalistic he was under Barcas's control and had no real combat expirence, all 5 of the other Enzyme II's showed to be very competente fighters and gave both Agito and Murakami a run for their money, heck the first three actulty beat Agito and would have killed him if not for everybodies favorite proto-zoalord.
While it is true that Enzyme III run on pure instinct it has served them very well as they have almost beaten Guyver I, II-F, III and Aptom in combat.