Ok man purple stem could be two things i think, right maybe its the start of a phosphourous diffincey as you said those girls are around 6-7 weeks old so they should be on bloom nutes now. Usually you will find purple stems when lacking in phosphorous.
One other thing is it might be getting too cold when lights go off in your tent this also cause's purple stems, sometimes its down to genetics with some autos but im not sure if thats the case.. id check the first two things first.
Also Sensi bloom a and b are very good nutes a little bit more pricey than some but they are woth it.