In many ways, it's actually the perfect entry for the Eldar. Eldar are masters of making their enemies fight each other until they're to weak to fend for themselves. If anything, the eldar are sitting on the sidelines manipulating things to ensure there is as much fighting as possible between the Rogue Traders. Things like False Astropathic messages, strike teams to sabotage ships before boarding actions, even lance or macrocannon fire from hidden ships aimed to look like they came from another Rogue Trader's ship.
Indeed, if they take long enough doing these battles (remember 1/2 hour turns et al, and cleaning up boarding actions can take even longer.) they might miss the window entirely for landing on the planet, and the Eldar will have won without loosing a single soldier.