Older cannondale roadies are known for having vertical dropouts and yours seem to have them. It could be a pain on your wallet to make that into a fixed gear build...

As for the saddle, I can go either way as far as comfort goes. But from a taste and style standpoint, concor's are one of my favorite saddles and it's a timeless design that can go on both modern and vintage bikes which I can't say for the arione.