*sigh* I really didn't want to get drawn into this...
Let's get one thing straight: the Migrant Fleet cannot contribute to the Reaper war effort in any way so long as their cargo holds are physically full with civilian housing. In their state before the Reaper war, they needed to stay close enough to a liveship for daily food deliveries, and were dependent on external infrastructure (which the Reapers quickly went about destroying) for basic survival. Logistics demand that they stay clumped together in a single mass which takes days to traverse a relay in order to keep everyone fed and protected. For their ships to be of use, they have to offload their civilians. That's non-negotiable.
There was little any faction could do against the Reapers other than buy time for the Crucible to be deployed. Any offensive against the Reapers, short of deploying the Crucible, was ultimately doomed to failure in the long term. Even the reinforcement of Palaven was at best temporary measure, with the Council species unable to hold the planet indefinitely. Time was an ally of the Reapers. With that as the backdrop logistics should not have been a major concern for the Quarians, as the Migrant Fleet would have been wasted if it were thrown against the Reapers in various offensive actions. The best use for the Migrant Fleet would be to hold them in reserve until the Crucible was ready to be deployed.
As a reserve the Migrant Fleet has the perfect setup as they aren't as dependent as other species on settled planets that the Reapers could seize. Fuel (HE3) can be skimmed from Gas Giants and water can be obtained from cracking uninhabited ice dwarfs. Food of course is all grown within the Live Ships.
The best course of action for the Quarians in keeping their civilians safe would be to keep them hidden in uninhabited systems, aboard the Migrant Fleet. Seizing Rannoch is ultimately foolish as it both ties the fleet down in now having to protect that planet against Geth or the Reapers, and puts the civilians directly in the path of a Reaper invasion. Had the Quarians settled Rannoch, the planet would have eventually shared the fates of Khar'Shan, Earth, Palaven, Irune, and Thessia. It gives the Reapers a target to attack.
There was no indication that negotiations between the Quarians and Geth would have borne fruit even if Legion hadn't severed comms before the Reapers invaded. Even if you bring Legion to the fleet, he says the Geth aren't willing to negotiate unless certain nebulously-defined conditions are met. Every diplomatic mission sent into Geth space for the last three centuries was shot down on sight and no effort was made by the Geth to distance themselves from the Heretics' actions. Prior to meeting Legion, all the information available to Shepard says the Geth are categorically Reaper allies - simply put, any Shepard who promotes peace talks before meeting Legion is an idiot.
Come ME3, Legion severed comms, putting an end to any notions of peace. The Reapers have invaded the galaxy, tearing down even military superpowers like the Turians. Meanwhile, the Quarians are bobbing along in an aging, less-than-maneuverable fleet, knowing that one Reaper entering the system they're in and taking potshots at the Liveships would spell extinction. They had to act in order to survive. But what options are open to them?
My problem with the Quarian Admirals isn't necessarily that they decided to go to war with the Geth. That peace was possible with the Geth requires meta knowledge that they didn't have, and it wasn't an unreasonable assumption for them to view the Geth as perpetually hostile. My issue with them is they couldn't have possibly chose a worse time to launch an invasion of their homeworld. The Quarians knew the Reapers were coming. They were the one faction (other than the Geth) who didn't doubt Shepard's claims. And yet instead of putting their war plans on the back burner they continue full speed ahead with Plan A, despite the Reapers invading Khar'Shan and Earth and Palaven. The Geth War was a colossal strategic blunder that threatened to bog their fleet down in a costly and potentially disastrous side show over Rannoch, when it could least be afforded.
Koris' stated plan was to hide in space, a state in which they cannot contribute to the Reaper war effort in any way ("We don't need a planet! The Migrant Fleet has survived for three centuries!" never mind it had eighty years left, tops, and the Reapers take centuries to sweep for survivors). Koris is a coward whose Reaper contingency plan would have resulted in the extinction of his species.
Gerrel and Xen, for all their faults, did what had to be done to ensure their species' survival while freeing up their fleet for the war effort at the same time. Rannoch is by no means immune to attack (nothing is) - once they've offloaded their civilians there, they're throwing in on the Crucible project like everyone else, depending on its success to save them. Would a peaceful resolution have been preferable? Of course. That was a failure on both sides of the table - Shepard's time would have been far better spent attempting mediation between them than jerking off in an Alliance holding cell.
Koris favored peace with the Geth, but he was no coward. His rescue mission illustrates that, where he'll suggest Shepard leave him to die and instead rescue his downed crewmen. I do agree that Koris is wrong about the long term viability of the Migrant Fleet, but he is absolutely right in seeking to avoid war with the Geth particularly while the Reaper War is raging. In that he was the only Quarian admiral (other than perhaps Tali, who was treated like a figurehead) to show any sense.
Han'Gerrel and Xen nearly blundered into destroying their own people, and Shala'Raan failed to help Koris avoid war despite not being on board with Han'Gerrel's plan. If any of the admirals was a coward, I think Shala'Raan best fits that bill. Perhaps not cowardice in the traditional sense, but she was a moral coward.