The only way to keep polycounts down is to start low, and stay low - it's far easier to split a poly into 2, than join 2 into 1. When making low poly models, you can put the detail where you need it - like in the face, you might decide to go for a chunky armour to cover the chest which will save polies because it can be low poly and still look right. Hands as well, you might want the right hand to have finders, and the left hand to be a fist - that way you can pose weapons really quickly - but a polygon reduction tool would probably kill the hand totally. It's a good idea to split your poly count:
Ideal count=1000
Arms x2=100 ea
Pelvis x1=100 ea
Legs x2=100 ea
Torso x1=250
Head x1=250
This isn't solid of course, but starting with a guide like that is good for working out things like cylinder segmets, like an arm might be made from a 6 sided cylinder with 7 sections, same for the legs - once you know your limits you know where you need to skimp, and with practice you'll find no end of tricks to make your life easier when character modelling. e.g. in my game Lonewolf, I barely game my characters a butt because you'd rarely see an enemy from behind, managed to keep the polycounts <700, that's why it can have 32 of them on screen at once without getting unplayable.
I would think that, if you wanted an end of level enemy, you could be a little more relaxed with the polycount, it all depends on what else the system has to do - so with end of level monsters, most of the other monsters are already gone, which free's up the necessary speed.
Van-B
It's c**p being the only coder in the village.