While I agree that alchemy is crazy overpowered and getting worse with the new rules, I don't know that moving where poison shields come from would make any difference in terms of the OPness of alchemy. Every shady PC would still need two packets to take down anyone playing the wrong race.
However, making poison shield a rogue skill may be problematic. First, in chapters I've played there have been few PC rogues. If 5% of the PC have access to poison shield, then things could get weird. But really, lots of players from different classes will be picking up poison shield because it would count toward their rogue build for buying dodges. So giving rogues poison shield probably doesn't even help rogues as much as it helps fighters (who are now buying dodges because they can't buy profs in the new rules).