I would definately not keep an adult 6" GSP in a 20gL. The bioload of these fish is tremendous & they love to swim around. Miine would have been thrilled to live as a singleton in a 55g.
When someone asks what they can keep in a 10g, I assume they mean for life, not "What can I start out or grow out in there?"
When someone asks what they can keep in a 10g, I assume they mean for life, not "What can I start out or grow out in there?"