Distance on transmission lines traces
To maintain the characteristic impedance, you should keep all other copper as far away from the transmission lines as possible.

Maintain a clearance of 10x the trace width if you can. 5x will probably be okay if you have to. 3x if you are willing to sacrifice signal integrity for design density.

Guard tracks are not necessary. If they are close enough to change the behavior of the transmission line, they will change the characteristic impedance. If they are far enough away to not affect the transmission line, then they could just as well be left off.