When you have a question like this that probably has been asked by many others already, try doing a search.
This popped up as the first reply when I searched.
https://ogrforum.ogaugerr.com/topic/aiu-cable-length
I have long runs of #14 stranded wire pairs running and it works well. There have been posts that actually suggest using extra wire on very short runs to equalize the signal and responses from the engines. I have short runs without any issues mixed with my long runs. I believe every layout presents different issues.
When I first read all the rules, I considered centering my single TIU (at the time) in the ceiling and run equal wires out. I tested with it at my power panel and never found any harm in mixed wires. My biggest problem was buying many engines and leaving them on powered track. That eventually dragged the signal down enough to cause issues. I had to install switches to kill the engine sidings so they all weren't on powered track. I later added extra TIU channels with the switches to keep things running smoothly.
