I've always kept my guinea pigs outside (owned a whole 3 in my life =P one when i was little so was a family pet, had Sega for 5+ years and Snes who only lasted a year =( )

but they were always outside, i would raise the cage a bit so it wasnt on the ground with the frost, and sometimes just put a tarp over the cage. I guess i never thought about the light cause they were outside. but i guess aslong as they get a good few hours of outside timeduring the day you can always bring them back in at night.

my family pet guinea pig Oliver got attacked by dogs one night and they didnt get through the cage, they bent it and cut themselves on it, but he was fine cause he could just hide. obviously not all pigs and dogs and cats are the same... but if you had them out while you were home you would hear if something was attacking them