I'd say use fixtures or don't. Don't mix it. You could just add your s1 and s2 to the fixtures. Otherwise just, don't load fixtures and use factories throughout your tests. I use fixtures only to populate the DB in my development env and factories in testing. See this article on the topic. Also for performance reasons, you should consider to use non-persisted (or even stubbed) objects where possbile, which is another advantage of factories over fixtures in testing.
Aside from the testing matter, I think you should use a scope instead of a class method