I'm wondering whether or not I'd be a negative influence to someone if my ascendant is conjunct their south node?
If anything it would be their
South Node influencing your Ascendant.
In traditional astrology South Node is unfavorable. In modern astrology it is the point where we are supposed to give out what we have taken in at the North Node (one of the interpretations and the easiest one i.m.o.).
Often where the South Node is placed in one's horoscope, it is associated with something we are already familiar with (from another life if you believe in karma)and which comes easy to us. Also it could make us choose for the "easy way out" I believe whereas the North Node is a "learning" point and where we have to put in some effort. So when the North Nodes point of growth is too much for us (to difficult for some reason) we could still use to stay comfortable with our South Node. There are all sorts of different theories about the Nodes.
Now if the person is giving out what he has learned via his North Node, than I guess you could benefit from it. That person could influence your personality by showing you something new maybe.
Or, you might find that person lazy and not really growing as a person and he or she could then maybe lean on you, being a bit of a bloodsucker maybe or wanting to "grow" with your help. I dont know, I am purely speculating as I find the Nodes interpretation quite multi facetted.
Maybe a traditional astrologer could give you more info on this position.