DC shouldnt cause too many issues - if any. If data gets corrupted, it isnt due to TCP/IP, it is really CSMA/CD Carrier Sense Multiple Access/Collision Detection. CSMA/CD is a layer 2 protocol in the OSI model, where TCP is Layer 3, and IP is layer 2. Anyway - I got off subject. Your goal is to keep collisions and data corruption to a minimum to keep traffic down.
I have used 1 cat5 cable to transmit 2 phone lines and 100bT connection and it is stable.
I would say youre probably OK using one cable, just make sure you keep track of your pairs! I would build a custom breakout cable / box and make note of what pairs are what. You dont want 24v being sent to your data circuit.
Good luck!
~Matt
