Yeah, that does bug me a bit (no pun intended) - the Kafers are somewhat limited in their expansion possibilities.
The problem could be fairly easily solved, I think, by making the Kafer homeworld at Psi Serpentis. So most of their expansion is "beyond the jump."
Within perhaps the last 50-100 years, the Suzerian on Psi Serpentis developed Stutterwarp tug technology utilizing Ylii slaves. Perhaps Lambda Serpentis turned out to be an ideal Kafer colony world, so there's been quite a bit of expansion across the gulf. Nevertheless, tug technology expensive and the new Kafer "states" on Lambda Serpentis side remain mostly on their own.
This opens up quite a bit of interesting possibility for the universe. The human forces can now take Gamma and Lambda Serpentis. The human vs. Kafer war would consist of the Kafer nations on the Lambda Serpentis side of the gap; which makes their defeat a lot more reasonable, it'd still probably involve giving the Kafers Arcturus and the finger reaching from there.
The Pentapods can do their thing on the heavily populated Lambda Serpentis. But now there's a gulf that standard Stutterwarp ships can't leap; the ultimate chokepoint. Neither side has the huge numbers of tugs to transport a battlefleet across the gulf and hope to defeat what's waiting on the other side. But both sides are keenly afraid that the other side will do just that. The humans have plenty of star systems to trade "land for time" while they muster their fleets. The Kafers are aware their homeworld is just on the other side of the gap and this rightfully terrifies them.