None at the moment.
Q: Can the hybrid walls connect to other walls?
A: Practically, they are connected. Visually, I had decided against that since it was inconsistent when I tested it, and was not nice to look at.
So, in short, no, I won't make them connect to other walls.
Q: Is this compatible with PvP or multiple player-factions?
A: It works, but currently all upgrades are global. I do have plans to separate upgrades per faction