In my experience only one enemy per mission can drop loot and it's only destroyed if an explosive deals the death blow. I pretty much always take two grenadiers, between the cover destruction, longer range on the launcher, and experimental grenades, I think they're probably the best all around class. Rangers are incredibly potent in the early game, so I usually field a second one of those for a while when I get my second squad size upgrade. Sharpshooters seem rather mediocre early on, I'm assuming they power up significantly in higher ranks.
Rank locked to SGT. I build GTS immediately after proving, so at worst I'm missing the first squad size upgrade for a mission, maybe two. Meanwhile I get my first experimental grenade finished and the skulljack started before the GTS even finishes. No point doing any ammo projects until you're about ready to switch to predator armor, you won't have the loadout space.