well after i did the quest and activated the Archimedes and killed the ncr troops, my rep with them went down to mixed. anyway i am against Caesar and his legion but when i went back to helios one a few days later i was suddenly attacked and i thought it was ncr but instead it was legionaries,
why would the legion come so far in ncr territory to get the powerplant

It's how the game is, one faction takes over if you kill another. I bet if you wipe out aother ncr place the same will happen. I wonder if they are at ranger station Charlie now.

They do take over ranger station charlie without you doing anything (talk to the ranger in Novac he asks you to check you go to charlie all NCR are dead and legion are there)

As to OP the more damage you do to NCR (remember activating killed the NCR there) the more the legion pushes in remember War War Never Changes.

oh yeah also the guy who is in charge of the helios one plant, mr fantastic is wearing legion armour and his other worker is dead so im thinking because i activated the laser this is what is meant to happen