What does Gene mean when he says, “Phineas, you wouldn’t be any good in the war, even if nothing had happened to your leg”? (190). What qualities make Finny a poor candidate for military service?

Gene might be of the opinion that Finny's lack of discipline and open enjoyment of breaking rules would have made him a bad candidate for the miltary. Finny liked to make his own rule, as well as break the rules others made. Gene might possibly be right, but I think Finny would have been fine no matter what he did.

“Finny,” my voice broke but I went on, “Phineas, you wouldn’t be any good in the war, even if nothing had happened to your leg.”
A look of amazement fell over him. It scared me, but I knew what I said was important and right, and my voice found that full tone voices have when they are expressing something long-felt and long-understood and released at last. “They’d get you some place at the front and there’d be a lull in the fighting, and the next thing anyone knew you’d be over with the Germans or the Japs, asking if they’d like to field a baseball team against our side. You’d be sitting in one of their command posts, teaching them English. Yes, you’d get confused and borrow one of their uniforms, and you’d lend them one of yours. Sure, that’s just what would happen. You’d get things so scrambled up nobody would know who to fight any more. You’d make a mess, a terrible mess, Finny, out of the war.”