Based on my reading of Shattered Fortress, I don't think you would have very long to recover POWs from the Capellans. But if you could, the FedSuns in particular would be a logical choice to do it. They are anti-Capellan, unlike Nikol's friendship with Danai, so Republic POWS would be more comfortable, have a long history of alliance with the Republic, and the Dawn Guards as an already existing regiment to add them to or base it on. As an extra bonus, Julian fought for the Republic, while Erik helped break it up. So they would likely be more pro-Julian forces for the inevitable power struggle.