I think civilian security already makes for a really good explanation for Gwen, Andy and Swanson. Since UK and France both aren't so much into military as the US, I could totally see them deciding that police work should be a civilian matter, the military shouldn't be given so much power on what is essentially a civilian-led operation (that did always annoy me a bit in the show, that they didn't even have a single civilian cop though they said it's a civilian-led expedition - I really liked how you handled that by adding civilian police in one of your Atlantis stories) So say, they separate normal base security, dealing with crimes within the crew as police work and military work as in off-world fighting missions, battling invading baddies and such.

Gwen could go because she's already in the know and as is-reassigned from Torchwood to the police formally (also, would Jack really want to leave re-building Torchwood after the whole team leaves for Pegasus in her hands?), someone from Torchwood could recommend Andy, having got to know him by way of Gwen (maybe even Gwen, not wanting to do all the police work in the Pegasus galaxy on her own.) And Detective Swanson is just that qualified that the upper ranks in the police have become aware of her good work and think of her when they're asked to recommend someone.