I tended to keep what ever ammo I had unless I had a problem with a particular enemy. After I died I would just load the game back up then switch the ammo.
Typically when you get farther in the game it becomes really easy, so when you get going at the game, turn it up a notch. There's so many options in that game, you can go back and play the entire thing on normal and have a different game entirely.
I always use armor piercing rounds. When you shoot and enemy you see their life bar. Occasionally some will have shields or armor (I believe they are blue and yellow bars, opposed to the normal red). The armor piercing rounds just mean that you do more damage to an enemies armor - in effect, they make your bullets more powerful. I never use chemical rounds; I never really needed them.
i have made it to the matriarch benezia,i think my skill level and squad is not strong enough and i get killed in a few seconds...i am really starting to hate this game. i actually looked up online some "hints" to get past that level but my crew vs the biotics of her people and the geth is a no contest. and i am real frustrated right now to the point im considering giving up on this game