Will it be possible give the FollowMe car a specific taxi route? This would be quite useful, as otherwise it won't be usable for online flight.
Surely not in the initial release, it will chose only the best path, but of course since it has to know the whole taxiway layout feature in any case, it's at least technically possible to add this feature.
Vehicles will prefer running over their dedicated lanes anyway so, they should be reasonably safe from clashing with other online airplanes which, as far as we are concerned, are just like any other AI.
However, I don't see how it would change anything regarding to online flying compatibility, as long as all the service vehicle aren't synchronized on the network, and they surely aren't (and we don't have any plans to do that, because it's really outside of the scope of the program), GSX vehicles will always be something local to your system, if only because all networked users should have GSX installed.