I wounder if it makes any difference when I activate the plasma purge on Noveria?
1. If I do it before fightning Benezia, Ventralis says he got orders from Benezia.
2. If Instead go through the door in the restricted area everyone goes hostile (which means that Ventralis will attack me if I got to him) and then fighting Benezia and do the plamsa purge after.
Which way is the most renegade and is there any difference in outcome by choosing one of those two options?
If you do the Neutron Purge before meeting the Queen, then Shepard's and the Queen's dialogue will change slightly because her children are already dead. Doing the Purge after meeting the Queen makes the dialogue a little wonky because Shepard has already found out from the Queen where she came from, so asking Petrovsky about where they came from is redundant.
I don't think either way is more "renegade." In fact, since Shepard is there looking for Benezia, it seems most logical to head right on down to the Hot Labs after Ventralis gives you the access card. Once down there you find out she is not there. (Liara will say "She's back at Rift Station.")
You forgot to mention the third possible way through which is the service entrance that Han Olar mentions if you meet him before going to the Hot Labs or trying to access the restricted area.
I don't believe it would effect renegade.
I usually activate the purge before talking with the Queen. The other times I don't activate the purge and go back to the Normandy.
If you are looking for the highest body count, setting off the Purge first will net you the highest kill score.
I go to the Hot Labs before Benezia typically, but that is just to maximize XP.
Actually the first few times I played the game I went through the maintenance tunnel to fight Benezia first and was surprised that the security turns hostile if you do it the other way. 
I usually don't bother with the Hot Labs at all.
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