The needler guns and the dart rifles both use darts, there is no difference, just like there's no difference between different small arm ammos.
I'm afraid not. The acid that's added to the dart is added by the Acid Dart Rifle itself.
An acid dart rifle has a nonreactive polymer reservoir that can be filled with whatever acid the wielder desires. The automatic loading process fills the rifle darts with acid as they are moved into the chamber. Dual acid dart rifles fire two darts simultaneously, while complex acid dart rifles hold concentrated doses of acid.
Since needler ammo is darts and acid dart ammo is darts, can acid dart acid be loaded in needler needles? If so, how much damage would they do?
Also, the base needler damage is 1d4 but the description also says it is a favorite of medics, so are there special darts that deliver medicines that don't do any damage?
Finally, pg 225 says serums can be imbibed or trickled down the throat and pg 190 under darts it says darts can be used to deliver healing serums...I assume injection works. Right?
The injection is what's doing the 1d4 damage as I understand it.