The levers you have should be compatible with Ultegra brakes. I think your reference to front brake only on singlespeeds only really applies to fixies. I've never seen a singlespeed with only one brake, just a fixie. If you are only using the bike as a singlespeed, keep the rear brake. It'll be safer. The only reason fixies only have a front brake, is because you can stop the wheel with your legs. If the brakes are at a good price, I'd get them.