Yes I would say you are probably spot-on there, Tink. Many future famous names did superb work in those programs.
However, those old SF's also supported many fine character actors who never really made it to the big time, too. One that comes to mind is Phyllis Thaxter (1919-2012), I always liked her especially. This photo is from 1955.
Learning a bit more about her because of your question, I discovered she had polio in 1952. Also, her close relationship with Montgomery Clift in the early 1940s ... that they "seemed so close that a great many people assumed they would eventually marry".
Nice question, I enjoyed it!