The less the object shows the less polygons need.

A cylinder does not need more edges where it is straight. If it is a small part that will not be seen close up the minium number of sides is 5 (a pentagon on each end). You might want more the bigger the part will be.

If the game system the model will go into supports normal maps then that is where the small detail can go instead of adding geometery. Even bump maps will do.

These are just a few ideas off the top of my head. I hope it helps.

Good luck.