RE: flech lenth
Yeah, 'just about' any broadhead available. The general rule of thumb has always been that feathers will stabilize a broadhead as well as vanes that are an inch longer. So a 4" feather will do the same job a 5" vane will. Unless you've got a really humongous broadhead, a 5" vane should stabilize it.
I've gotten 75 grain Thunderheads to shoot great with 2" Bohning Blazer vanes, but any head bigger than that went crazy. With some of those really huge 2-blade heads, like the 2" wide Simmons Tree Shark, I've had to go to 5 1/2" magnum feathers to get 'em stabilized.