Questions about poison

LF help about some questions bothering me since POE1 about poison:

1. some talents making DoT damage faster, how does this work? does it make poisons doing same amount of damage in less than 2 seconds or it still last 2 seconds but the periodic is now less than 1 sec per tick?

2. do skills like GAS Arrow still need chance on hit to poison? since they already guaranteed poisoning?