You can also get 1 oz factory loads in 2 1/2" hulls, specifically made for Brit guns (so low pressure) from Kent Gamebore. They list the Pure Golds as being available in 5 shot. However, those may be British shot sizes, which means their 5's are basically our 6's.

If I were you, with just an ounce of shot, I don't think I'd go anything larger than 5's. They'll do a good job on December roosters and will give you better pattern density than 4's.