Remember that the background is moving along around you, so perhaps Capital ships can only move at the speed of the subspace tunnel (I.e. 0 in subspace) while smaller fighters are capable of manuvering due to their small size.
Or, maybe a capital ship is the centre of the subspace tunnel we see, with their reactors being the source of the subspace disturbance (in this case, the Lucifer was). If so, then maybe strike craft (fighters and bombers) can actually "surf" the tunnels of larger vessels.
As for the Gargant thing, there's is no such thing as an undefeatable enemy. It'll be dealt with one way or another. As for how, i'll just wait and see what the Inferno team comes up with.