Goldfish will not be an effective method of reducing the weed and algae from your pond,for the little they will eat.
You really need to get the balance right by using other methods, although grass carp will eat weed especially duck weed.
Algae will grow vigorously in strong sun-light, so shading it out will kill it off, you can buy products that can controll algael growth from aquatic centres, which I think mostly work on the principal of clouding the water to filter-out UV rays, small bails of berley straw are also available that inhibit algae as the straw breaks down and decays.
Plants like lily pads will also cut out much of the sun-light, if you aim at keeping approx 75% of surface area covered with them, there are several miniature varieties suitable for small ponds.
Personally if you have a lot of weed in there, especially blancket weed, I'd get this under control first before adding any fish. Good Luck. Tbird+