Hmm, it would appear I've been neglecting... STOP THAT DRAIGNEAN, OR I'LL SET YOUR CAT ON FIRE.
If a suit was put on James, would the fungus and the suit fight another?
Fight is the wrong word, but as long as James' brain was connected to the suit the fungus would be capable of spreading into the bio-suit's brain(s). Without empathetics (or Technocrat high-technology) "fighting" is almost impossible. I only say almost because anything is possible.
My head hurts, should I be worried?
Your condition is too vague to diagnose, it could be very minor, or you could die in the next five minutes. Just ignore it for now, if you start bleeding from your eyes/mouth/ears/nose or hearing phantom voices, then you should take a break from the computer and go outside.
If James wins, can he expect any rewards at all? (not looking for specifics, just yes or no).
Yes. But how much and what are entirely dependent upon how, and how well, he succeeds. He could also suffer debilitating status effects (like the matriarch) in addition to his reward.
MY COMPUTER IS COVERED WITH INSECTS!!!
Our time is up, the age of man is at an end, the era of the insect has arrived. Our only hope is to join the arachnid resistance forces.
Can James fight a suit in his mind if one is put on him, since he's already fighting the fungus?
Yes, but again, fight isn't exactly the right word. The fungus isn't exactly fighting James, at least not anymore than a tree is fighting a signpost as it grows through it.
Fighting the suit would be a very different war, one that the suit would be at a significant disadvantage to fight.