...on neutral planets alone?

...on enemy planets alone?

Sure they can land, but you won't gain control of the planet.

And if I destroyed all the enemy GF, what happens to a mercenary?

He/she stays there, but the planet becomes neutral. Merc is yours, but is not GF so can't capture the planet. Can be useful if you don't want to commit a main force in fear of retaliation, but otherwise better to send in GF's with them.