I find the best thing to do with greenfly is to brush them off with my fingers (or even a soft brush if you're squeamish) - leaving a few for the bluetits and ladybirds. I did this to mine a couple of times afew weeks ago and there are hardly any greenfly there and certainly no damage on the roses.
As for feeding them, I mulch with well rotted farm yard manure in the autumn and top dress with Fish Blood and Bone in the spring, with maybe a little more no later than late July.
As for the rust spots, is this black spot? There's not a lot you can do about this other than ensure you rake up dropped leaves in the autumn and destroy them. Some roses are more susceptible to blackspot than others. It used to be less common, when the air was full of sulphur smog from coal fires. It's the lovely clean air that we breathe nowadays that lets the blackspot spread. Swings and roundabouts!