You can buy chest armour that has some shoulder protection. 661 has one as does every company I believe. I don't think you can JUST have shoulder. The chest armour is small and light and only offers the smallest protection so I don't imagine it is too cumbersome.
Giros are expensive. There are cheaper brands out there. Netti and Mace both have helmets that are 140 cdn. Cheap, protective AND good ventilation may be another issue.