I may be missing something, but was this a planned actions by the guys who attacked her? Or was it that she happened to be separated from her squad and they just happened upon her at took advantage of her separation?

No, they weren't in her squad. The point was specifically made that she had been separated from her Squad.

She knew exactly who they were because they were in her squad. She named them all to her psychiatrist which is how Stowe's character found out about them later on.
You need to rewatch the film. The shrink let them in his office and pointed them towards another soldier's file. He was the one who described the rape, meaning he was there, then Madeline uses a really lame ruse to trick him into identifying the other perps.

She knew exactly who they were because they were in her squad. She named them all to her psychiatrist which is how Stowe's character found out about them later on.

She had no way to identify her attackers. It was dark, they were wearing camo and ghilli suits, it was dark, and she was seperated from her unit (i.e. people she knew). She really didn't have any way to definitely identify who did it, and they would know that. In the novel, there were also a bunch of soldiers from Ft. Bragg taking part of the maneuvers, so there were several thousand people out in the field that night. They just figured she'd never be able to identify them, or that she be too afraid to tell after they threatened her.
Whores will have their trinkets.